A consultant (from the Latin consultare meaning "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 accountancy, the environment, technology, law, human resources, marketing, medicine, finance, public affairs, communication, engineering or waste management.
The main difference between a consultant and a 'normal' expert is that the consultant is not himself employed with his client, but instead is in business for himself or for a consultancy firm, usually with multiple and changing clients. Thus, his clients have access to deeper levels of expertise than would be feasible for them to retain in-house, especially if the speciality is needed comparatively rarely. It is generally accepted good corporate governance to hire consultants as a check to the Principal-Agent problem.
A consultant is also the term used to denote the most senior medical position in the United Kingdom (e.g., a consultant surgeon).