A consultant (from the Latin consultare means "to discuss" from which we also derive words such as consul and counsel) is a professional who provides advice in a particular area of expertise such as management, accountancy, the environment, entertainment, technology, law (tax law, in particular), human resources, marketing, medicine, finance, life management, economics, public affairs, communication, engineering, sound system design, graphic design, or waste management.
A consultant is usually an expert or a professional in a specific field and has a wide knowledge of the subject matter. A consultant usually works for a consultancy firm or is self-employed, and engages with multiple and changing clients. Thus, clients have access to deeper levels of expertise than would be feasible for them to retain in-house, and may purchase only as much service from the outside consultant as desired. It is generally accepted good corporate governance to hire consultants as a check to the Principal-Agent problem.
'Consultant' is also the term used to denote the most senior medical position in the United Kingdom, Australia and Ireland (e.g., a consultant surgeon).