An angel investor (also known as a business angel , informal investor , angel funder , private investor , or seed investor ) is an affluent individual who provides capital for a business start-up, usually in exchange for convertible debt or ownership equity. A small but increasing number of angel investors invest online through equity crowdfunding or organize themselves into angel groups or angel networks to share research and pool their investment capital, as well as to provide advice to their portfolio companies. 
Etymology and origin
The application of the term "angel" to a kind of investor originally comes from Broadway theater, where it was used to describe wealthy individuals who provided money for theatrical productions that would otherwise have had to shut down. In 1978, William Wetzel, then a professor at the University of New Hampshire and founder of its Center for Venture Research, completed a pioneering study on how entrepreneurs raised seed capital in the USA, and he began using the term "angel" to describe the investors that supported them. A similar term is patron, commonly used in arts.
Angel investors are often retired entrepreneurs or executives, who may be interested in angel investing for reasons that go beyond pure monetary return. These include wanting to keep abreast of current developments in a particular business arena, mentoring another generation of entrepreneurs, and making use of their experience and networks on a less than full-time basis. Thus, in addition to funds, angel investors can often provide valuable management advice and important contacts. Because there are no public exchanges listing their securities, private companies meet angel investors in several ways, including referrals from the investors' trusted sources and other business contacts; at investor conferences and symposia; and at meetings organized by groups of angels where companies pitch directly to investor in face-to-face meetings.
According to the Center for Venture Research, there were 258,000 active angel investors in the U.S. in 2007.  According to literature reviewed by the US Small Business Administration, the number of individuals in the US who made an angel investment between 2001 and 2003 is between 300,000 and 600,000.  Beginning in the late 1980s, angels started to coalesce into informal groups with the goal of sharing deal flow and due diligence work, and pooling their funds to make larger investments. Angel groups are generally local organizations made up of 10 to 150 accredited investors interested in early-stage investing. In 1996 there were about 10 angel groups in the United States. There were over 200 as of 2006. 
The past few years, particularly in North America, have seen the emergence of networks of angel groups, through which companies that apply for funding to one group are then brought before other groups to raise additional capital.
Source and extent of funding
Angels typically invest their own funds, unlike venture capitalists who manage the pooled money of others in a professionally managed fund.   Although typically reflecting the investment judgment of an individual, the actual entity that provides the funding may be a trust, business, limited liability company, investment fund, or other vehicle. A Harvard report  by William R. Kerr, Josh Lerner, and Antoinette Schoar provides evidence that angel-funded startups are more likely to succeed than companies that rely on other forms of initial financing. The paper by Kerr et al., found "that angel funding is positively correlated with higher survival, additional fundraising outside the angel group, and faster growth measured through growth in web site traffic".
Angel capital fills the gap in seed funding between "friends and family" and more robust start-up financing through formal venture capital. Although it is usually difficult to raise more than a few hundred thousand dollars from friends and family, most traditional venture capital funds are usually not able to make or evaluate small investments under US$1–2 million. Thus, angel investment is a common second round of financing for high-growth start-ups, and accounts in total for almost as much money invested annually as all venture capital funds combined, but into more than 60 times as many companies (US$20.1 billion vs. $23.26 billion in the US in 2010, into 61,900 companies vs. 1,012 companies).  
There is no "set amount" for angel investors, and the range can go anywhere from a few thousand, to a few million dollars. In a large shift from 2009, in 2010 healthcare/medical accounted for the largest share of angel investments, with 30% of total angel investments (vs. 17% in 2009), followed by software (16% vs. 19% in 2007), biotech (15% vs. 8% in 2009), industrial/energy (8% vs. 17% in 2009), retail (5% vs. 8% in 2009) and IT services (5%).   While more readily available than venture financing, angel investment is still extremely difficult to raise.  However some new models are developing that are trying to make this easier. 
Angel investments bear extremely high risks and are usually subject to dilution from future investment rounds. As such, they require a very high return on investment. Because a large percentage of angel investments are lost completely when early stage companies fail, professional angel investors seek investments that have the potential to return at least ten or more times their original investment within 5 years, through a defined exit strategy, such as plans for an initial public offering or an acquisition. Current 'best practices' suggest that angels might do better setting their sights even higher, looking for companies that will have at least the potential to provide a 20x-30x return over a five- to seven-year holding period. After taking into account the need to cover failed investments and the multi-year holding time for even the successful ones, however, the actual effective internal rate of return for a typical successful portfolio of angel investments is, in reality, typically as 'low' as 20–30%. While the investor's need for high rates of return on any given investment can thus make angel financing an expensive source of funds, cheaper sources of capital, such as bank financing, are usually not available for most early-stage ventures.
Geographically, Silicon Valley dominates United States angel investing, receiving 39% of the $7.5B invested in US-based companies throughout Q2 2011, 3–4 times as much as the total amount invested within New England.  Total investments in 2011 were $22.5 billion, an increase of 12.1 percent over 2010 when investments totalled $20.1 billion.  In the United States, angels are generally accredited investors in order to comply with current SEC regulations, although the JOBS Act of 2012 loosened those requirements starting in January 2013. Reaching nearly $23 billion in 2012 in the US, angel investors are not only responsible for funding over 67,000 startup ventures annually, but their capital also contributed to job growth by helping to finance 274,800 new jobs in 2012.  In 2013, 41% of tech sector executives name angel investors as a means of funding.
A study by NESTA  in 2009 estimated that there were between 4,000 and 6,000 angel investors in the UK with an average investment size of £42,000 per investment. Furthermore, each angel investor on average acquired 8 percent of the venture in the deal with 10 percent of investments accounting for more than 20 percent of the venture.
In terms of returns, 35 percent of investments produced returns of between one and five times of the initial investment, whilst 9 percent produced returns of multiples of ten times or more. The mean return, however, was 2.2 times investment in 3.6 years and an approximate internal rate of return of 22 percent gross.
The UK Business Angel market grew in 2009/2010 and, despite recessionary concerns, continues to show signs of growth.   In 2013, this dynamic kept going on in the UK as angel investors were named by two-thirds of technology entrepreneurs as a means of funding. By 2015, angel investments had increased throughout the UK, with the average number of investments made by angels at 5, compared to 2.5 in 2009. The same report also found an increase in angel investors making impact investments, with 25% of angels saying they had made an impact investment in 2014. 
Fifteen years ago, it was difficult for startups in China to find local angel investors, but that has changed.  Angel groups such as Angelvest needed to be created from scratch by David Chen and media executive Ted Lai. Billionaire tech entrepreneur and grassroots investor Cai Wensheng also founded Long Neck Investments.