Infrastructure is the fundamental facilities and systems serving a country, city, or other area,^[1] including the services and facilities necessary for its economy to function.^[2] Infrastructure is composed of public and private physical improvements such as roads, railways, bridges, tunnels, water supply, sewers, electrical grids, and telecommunications (including Internet connectivity and broadband speeds). In general, it has also been defined as "the physical components of interrelated systems providing commodities and services essential to enable, sustain, or enhance societal living conditions".^[3]

There are two general types of ways to view infrastructure, hard or soft. Hard infrastructure refers to the physical networks necessary for the functioning of a modern industry.^[4] This includes roads, bridges, railways, etc. Soft infrastructure refers to all the institutions that maintain the economic, health, social, and cultural standards of a country.^[4] This includes educational programs, official statistics, parks and recreational facilities, law enforcement agencies, and emergency services.

The word infrastructure has been used in French since 1875 and in English since 1887, originally meaning "The installations that form the basis for any operation or system".^[5][6] The word was imported from French, where it was already used for establishing a roadbed of substrate material, required before railroad tracks or constructed pavement could be laid on top of it. The word is a combination of the Latin prefix "infra", meaning "below" and many of these constructions are underground, for example, tunnels, water and gas systems, and railways and the French word "structure" (derived from the Latin word "structura"). The army use of the term achieved currency in the United States after the formation of NATO in the 1940s, and by 1970 was adopted by urban planners in its modern civilian sense.^[7]

A 1987 US National Research Council panel adopted the term "public works infrastructure", referring to:

The American Society of Civil Engineers publish a "Infrastructure Report Card" which represents the organizations opinion on the condition of various infrastructure every 2–4 years.^[9] As of 2017 they grade 16 categories, namely aviation, bridges, dams, drinking water, energy, hazardous waste, inland waterways, levees, parks & recreation, ports, rail, roads, schools, solid waste, transit and wastewater.^{[9] []}

A way to embody personal infrastructure is to think of it in term of human capital.^[10] Human capital is defined by the Encyclopædia Britannica as “intangible collective resources possessed by individuals and groups within a given population".^[11] The goal of personal infrastructure is to determine the quality of the economic agents’ values. This results in three major tasks: the task of economic proxies’ in the economic process (teachers, unskilled and qualified labor, etc.); the importance of personal infrastructure for an individual (short and long-term consumption of education); and the social relevance of personal infrastructure.^[10]

Institutional infrastructure branches from the term "economic constitution". According to Gianpiero Torrisi, Institutional infrastructure is the object of economic and legal policy. It compromises the grown and sets norms.^[10] It refers to the degree of actual equal treatment of equal economic data and determines the framework within which economic agents may formulate their own economic plans and carry them out in co-operation with others.

Material infrastructure is defined as “those immobile, non-circulating capital goods that essentially contribute to the production of infrastructure goods and services needed to satisfy basic physical and social requirements of economic agents".^[10] There are two distinct qualities of material infrastructures: 1) Fulfillment of social needs and 2) Mass production. The first characteristic deals with the basic needs of human life. The second characteristic is the non-availability of infrastructure goods and services.^[10]

According to the business dictionary, economic infrastructure can be defined as "internal facilities of a country that make business activity possible, such as communication, transportation and distribution networks, financial institutions and markets, and energy supply systems".^[12] Economic infrastructure support productive activities and events. This includes roads, highways, bridges, airports, cycling infrastructure, water distribution networks, sewer systems, irrigation plants, etc.^[10]

Social infrastructure can be broadly defined as the construction and maintenance of facilities that support social services.^[13] Social infrastructures are created to increase social comfort and act on economic activity. These being schools, parks and playgrounds, structures for public safety, waste disposal plants, hospitals, sports area, etc.^[10]

Core assets provide essential services and have monopolistic characteristics.^[14] Investors seeking core infrastructure look for five different characteristics: Income, Low volatility of returns, Diversification, Inflation Protection, and Long-term liability matching.^[14] Core Infrastructure incorporates all the main types of infrastructure. For instance; roads, highways, railways, public transportation, water and gas supply, etc.

Basic infrastructure refers to main railways, roads, canals, harbors and docks, the electromagnetic telegraph, drainage, dikes, and land reclamation.^[10] It consist of the more well-known features of infrastructure. The things in the world we come across everyday (buildings, roads, docks, etc.).

Complementary infrastructure refers to things like light railways, tramways, gas/electricity/water supply, etc.^[10] To complement something, means to bring to perfection or complete it. So, complementary infrastructure deals with the little parts of the engineering world the brings more life. The lights on the sidewalks, the landscaping around buildings, the benches for pedestrians to rest, etc.

The term infrastructure may be confused with the following overlapping or related concepts.

Land improvement and land development are general terms that in some contexts may include infrastructure, but in the context of a discussion of infrastructure would refer only to smaller scale systems or works that are not included in infrastructure, because they are typically limited to a single parcel of land, and are owned and operated by the land owner. For example, an irrigation canal that serves a region or district would be included with infrastructure, but the private irrigation systems on individual land parcels would be considered land improvements, not infrastructure. Service connections to municipal service and public utility networks would also be considered land improvements, not infrastructure.^[15][16]

The term public works includes government-owned and operated infrastructure as well as public buildings, such as schools and court houses. Public works generally refers to physical assets needed to deliver public services. Public services include both infrastructure and services generally provided by government.

Infrastructure may be owned and managed by governments or by private companies, such as sole public utility or railway companies. Generally, most roads, major airports and other ports, water distribution systems, and sewage networks are publicly owned, whereas most energy and telecommunications networks are privately owned. Publicly owned infrastructure may be paid for from taxes, tolls, or metered user fees, whereas private infrastructure is generally paid for by metered user fees. Major investment projects are generally financed by the issuance of long-term bonds.

Government-owned and operated infrastructure may be developed and operated in the private sector or in public-private partnerships, in addition to in the public sector. As of 2008 in the United States for example, public spending on infrastructure has varied between 2.3% and 3.6% of GDP since 1950.^[17] Many financial institutions invest in infrastructure.

Engineers generally limit the term "infrastructure" to describe fixed assets that are in the form of a large network; in other words, hard infrastructure. Efforts to devise more generic definitions of infrastructures have typically referred to the network aspects of most of the structures, and to the accumulated value of investments in the networks as assets. One such definition from 1998 defined infrastructure as the network of assets "where the system as a whole is intended to be maintained indefinitely at a specified standard of service by the continuing replacement and refurbishment of its components".^[18]

Civil defense planners and developmental economists generally refer to both hard and soft infrastructure, including public services such as schools and hospitals, emergency services such as police and fire fighting, and basic financial services. The notion of infrastructure-based development combining long-term infrastructure investments by government agencies at central and regional levels with public private partnerships has proven popular among economists in Asia (notably Singapore and China), mainland Europe, and Latin America.

Military infrastructure is the buildings and permanent installations necessary for the support of military forces, whether they are stationed in bases, being deployed or engaged in operations. For example, barracks, headquarters, airfields, communications facilities, stores of military equipment, port installations, and maintenance stations.^[19]

Communications infrastructure is the informal and formal channels of communication, political and social networks, or beliefs held by members of particular groups, as well as information technology, software development tools. Still underlying these more conceptual uses is the idea that infrastructure provides organizing structure and support for the system or organization it serves, whether it is a city, a nation, a corporation, or a collection of people with common interests. Examples include IT infrastructure, research infrastructure, terrorist infrastructure, employment infrastructure and tourism infrastructure.

According to researchers at the Overseas Development Institute, the lack of infrastructure in many developing countries represents one of the most significant limitations to economic growth and achievement of the Millennium Development Goals (MDGs). Infrastructure investments and maintenance can be very expensive, especially in such areas as landlocked, rural and sparsely populated countries in Africa. It has been argued that infrastructure investments contributed to more than half of Africa's improved growth performance between 1990 and 2005, and increased investment is necessary to maintain growth and tackle poverty. The returns to investment in infrastructure are very significant, with on average thirty to forty percent returns for telecommunications (ICT) investments, over forty percent for electricity generation, and eighty percent for roads.^[20]

The demand for infrastructure, both by consumers and by companies is much higher than the amount invested.^[20] There are severe constraints on the supply side of the provision of infrastructure in Asia.^[21] The infrastructure financing gap between what is invested in Asia-Pacific (around US$48 billion) and what is needed (US$228 billion) is around US$180 billion every year.^[20]

In Latin America, three percent of GDP (around US$71 billion) would need to be invested in infrastructure in order to satisfy demand, yet in 2005, for example, only around two percent was invested leaving a financing gap of approximately US$24 billion.^[20]

In Africa, in order to reach the seven percent annual growth calculated to be required to meet the MDGs by 2015 would require infrastructure investments of about fifteen percent of GDP, or around US$93 billion a year. In fragile states, over thirty-seven percent of GDP would be required.^[20]

The source of financing varies significantly across sectors. Some sectors are dominated by government spending, others by overseas development aid (ODA), and yet others by private investors.^[20] In California, infrastructure financing districts are established by local governments to pay for physical facilities and services within a specified area by using property tax increases.^[22] In order to facilitate investment of the private sector in developing countries' infrastructure markets, it is necessary to design risk-allocation mechanisms more carefully, given the higher risks of their markets.^[23]

The spending money that comes from the government is less than it used to be. Compared to the global GDP percentages, The United States is tied for second-to-last place, with an average percentage of 2.4%. This means that the government spends less money on repairing old infrastructure and or on infrastructure as a whole.^[24]

In Sub-Saharan Africa, governments spend around US$9.4 billion out of a total of US$24.9 billion. In irrigation, governments represent almost all spending. In transport and energy a majority of investment is government spending. In ICT and water supply and sanitation, the private sector represents the majority of capital expenditure. Overall, between them aid, the private sector, and non-OECD financiers exceed government spending. The private sector spending alone equals state capital expenditure, though the majority is focused on ICT infrastructure investments. External financing increased in the 2000s (decade) and in Africa alone external infrastructure investments increased from US$7 billion in 2002 to US$27 billion in 2009. China, in particular, has emerged as an important investor.^[20]