A consultant is usually an expert or an experienced professional in a specific field and has a wide knowledge of the subject matter. 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:
Internal consultant: someone who operates within an organization but is available to be consulted on areas of their specialization by other departments or individuals (acting as clients).
External consultant: someone who is employed externally to the client whose expertise is provided on a temporary basis, usually for a fee. Consulting firms range in size from sole proprietorship consisting of a single consultant, small businesses consisting of a small number of consultants, to mid- to large consulting firms, which in some cases are multinational corporations. This type of consultant generally engages with multiple and changing clients, which are typically companies, non-profit organizations, or governments.
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. As well, clients can control their expenditures on consulting services by only purchasing as much services from the outside consultant as desired.