A consultant is a professional who provides professional or expert advice in a particular area such as management, accountancy, the environment, entertainment, technology, law (tax law, in particular), human resources, marketing, emergency management, food production, 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.