A consultant is usually an expert or a 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 specialism by other departments or individuals (acting as clients); or
External Consultant: Someone who is employed externally (either by a firm or some other agency) whose expertise is provided on a temporary basis, usually for a fee. As such this type of consultant generally engages with multiple and changing clients.
The overall impact of a consultant is that 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.