An expert who gives advice

An expert who is called in to advise a business or other organization about various aspects of its operations.
A computer professional who deals with client firms as an independent contractor rather than as an employee. Consultants are often engaged to analyze user needs and develop system specifications.

A quasi-judicial officer appointed by a court to take testimony or hear specified types of cases. These officials are sometimes called masters.
In science, one who reads and comments on a scientific paper before its publication, normally a scientist of at least equal standing to the author(s).