A formal group of experts (or even a single expert) brought together on a regular or ad hoc basis to investigate or debate matters within that sphere of expertise, and with regulatory or quasi-judicial powers such as the ability to license activity in the sphere of activity or to subpoena witnesses.
Commissions usually also have advisory powers to government.
The organizational form of a commission is often resorted to by governments to exhaustively investigate a matter of national concern, and is often known as a "commission of inquiry."
This legal structure can be contrasted with quasi-judicial or regulatory powers.