(Sports) The chief official (as in boxing or American football) who is expected to ensure fair play; SYN. ref.

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).

1. To be a referee or umpire; SYN. umpire.
2. To evaluate professionally a colleague's work; SYN. peer review.