A consultant (from Latin: consultare "to deliberate") is a professional who provides expert advice^[1] in a particular area such as security (electronic or physical), management, education, Energy^[9], accountancy, law, human resources, marketing (and public relations), finance, health care, engineering, science or any of many other specialized fields.

A consultant is usually an expert or an experienced professional in a specific field and has a wide knowledge of the subject matter.^[2] The role of consultant outside the medical sphere (where the term is used specifically for a grade of doctor) can fall under one of two general categories:

By hiring a consultant, clients have access to deeper levels of expertise than would be financially feasible for them to retain in-house on a long-term basis.

Moreover, clients can control their expenditures on consulting services by only purchasing as much services from the outside consultant as desired.

Consultants provide their advice to their clients in a variety of forms.

Reports and presentations are often used.

However, in some specialized fields, the consultant may develop customized software or other products for the client.

Depending on the nature of the consulting services and the wishes of the client, the advice from the consultant may be made public, by placing the report or presentation online, or the advice may be kept confidential, and only given to the senior executives of the organization paying for the consulting services^[3].

The range of areas of expertise covered by the term "consultant" is wide.

One of the more common types is the management consultant. Consulting and the means by which the (external) consultant is engaged vary according to industry and local practice. However the principal difference between a consultant and atemp is generally one of direction. A consultant or temp is engaged to fulfill a brief in terms of helping to find solutions to specific issues but the ways in which that is to be done generally falls to the consultant to decide. An information systems or project management consultant is also referred as just a consultant who manages constraints such as budget and resources agreed with the client. An external consultant, on the other hand is normally fulfilling a non-employee role that usually exists within the organization and is helping to bridge a gap caused by staffing shortages, skills and expertise. They are directed by the normal management structure of the organization. There is, however, a hybrid form where a consultant may be hired as an interim manager or executive, bringing a combination of specialist expertise to bear on a role that is temporarily vacant (usually at a senior level).

A second difference is that temp is generally used for labor-oriented work whereas an external consultant is generally used for service-oriented work.

Consultants and temps are those that work for clients.

Both of them are non-employees of an organization and both work on the basis of contract terms.

Some companies have employees of the company act as internal consultants and they provide cross-team advice.

In most cases, however, employees of a company titled as consultants are those that work with the clients of that company and are external to the client.

A manager at the client company, to whom the consultant or temp reports, does not have direct authority or responsibility over the outcome of the consultant's work because they are external and are providing a service to that company.

As long they are external to the company/team they are consultants, but as soon as they join the company/team they become employees/team members and are given job titles based on their skills.

Some consultants are employed indirectly by the client via a consultancy staffing company, a company that provides consultants on an agency basis.

The staffing company itself does not usually have consulting expertise but works rather like an employment agency.

This form of working is particularly common in the ICT sector. Such consultants are often called "contractors" since they are usually providing technical services (such as programming or systems analysis) that could be performed in-house were it not easier for the employer to operate a flexible system of only hiring such technologists at times of peak workload rather than permanently.

While many consultants work for firms, there is also an increasing number of independent consultants. Many of these professionals also join networks or alliances that allow them to find collaborators and new clients.

In the business, and as of recently the private sphere, the most commonly found consultants are:

A more comprehensive list of types is shown below.

Though most of the back-office research and analysis occurs at the consultants' offices or home-offices, in the case of smaller consulting firms, consultants typically work at the site of the client for at least some of the time. By spending time at the client's organization, the consultant is able to observe work processes, interview workers, managers, executives, board members, or other individuals, and study how the organization operates.

The governing factor on where a consultant works tends to be the amount of interaction required with other employees of the client.

If a management consultant is providing advice to a software firm that is struggling with employee morale, absenteeism and issues with managers and senior engineers leaving the firm, the consultant will probably spend a good deal of time at the client's office, interviewing staff, engineers, managers and executives, and observing work processes. On the other hand, a legal consultant asked to provide advice on a specific property law issue might only have a few meetings at the client's office, and conduct the majority of his work at the consultant's office and in legal libraries.

Similarly, the growth of online, highly skilled consultant marketplaces has begun to grow.^[4] These online platforms provide consultants with experience working for typical consulting firms to easily transition into freelancing. This means that many consultants have become much more flexible in where they can work and the nature of their work.

There is no single qualification to becoming a consultant, other than those laid down in relation to medical, psychological and engineering personnel who have attained this level-degree in it or professional licenses.

Consultants may hold undergraduate degrees, graduate degrees, professional degrees or professional designations pertaining to their field(s) of expertise. In some fields, a consultant may be required to hold certain professional licenses (e.g., a civil engineer providing consulting on a bridge project may have to be a professional engineer). In other types of consulting, there may be no specific qualification requirements. A legal consultant may have to be a member of the bar or hold a law degree. An accounting consultant may have to have an accounting designation, such as Chartered Accountant status. On the other hand, some individuals become consultants after a lengthy and distinguished career as an executive or political leader, so their management or government experience may be their main "credential", rather than a degree or professional designation.

Consultant Peter Block defines a consultant as "someone who has influence over an individual, group, or organization, but who has no direct authority to implement changes." He contrasts this with a surrogate manager who is a person who "acts on behalf of, or in place of, a manager." The key difference is that a consultant never makes decisions for the individual or group, whereas a surrogate manager does make decisions.

Accredited associates are bound by a Code of Ethics that require the consultant to only provide “practical advice that works”—“Analysing as a Generalist and Solving as a Specialist”—by using the skills and experience of a sub-contracted fellow Associate, thus at all times providing the client with the best available advice and support. Internationally the accreditation of management consultants is overseen by higher education training and accreditation organizations.

For management consultancy services, the ISO 20700 standard has been available since 2017.