A consultant (from Latin: consultare "to deliberate") is a professional (also known as expert, specialist, see variations of meaning below) who provides advice or services in an area of specialization (generally to medium or large-size corporations). Consulting services generally fall under the domain of professional services, as contingent work.

The Harvard Business School defines a consultant as someone who advises on "how to modify, proceed in, or streamline a given process within a specialized field".

According to Institute of Management Consultants USA, "The value of a consultant [as compared to a subject-matter expert (SME)] is to be able to correctly diagnose and effectively transform an often ill-defined problem and apply information, resources and processes to create a workable and usable solution. Some experts are good consultants and vice versa, some are neither, few are both." Another differentiation would be that a consultant sells advice, whereas an expert sells their expertise. Other differentiations exist for consultants vs. coaches or SMEs vs. team leaders.

Consultants do not have to be subject-matter experts as consulting agreements are a form of labor contract - comparable to staffing, which a client procures for more generalized labor, whereas consulting is for more specialized labor. Thus, in contrast to advisory services, which is not a labor contract but an actual service (advisory services never become part of the procuring organization) the market for consulting agreements follows the demand for specialized labor in the form of a consulting procurement, and so while competence and experience is naturally an advantage for when looking to sell consulting services, it is not a prerequisite in the same way that it is for advisory services where the service provider per definition relies on some level of competence and experience for its relevance.

Sometimes, the word consultant applies specifically to someone or organization that provides knowledge, advice or service; whereas the contractor builds something for the client.

The role of a 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 procuring consulting services, clients may acquire access to higher grades 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. Additionally, consultants are key persons with specific domain-skills in creating strategies, leading change (e.g. digitalization), leadership coaching, interim management (also called consultant manager), etc.

Another business-case is that a consultant may save the company money: for example, a specialist tax-consultant who saves the company 20% on its taxes, and only charges 10% in fees, enables the company to net a 10% savings. A portion of professional services in demand for clients are simply not necessary to retain in house, as they may be sporadic in nature, at which a consultant offers a reduction in payroll for the client.