What is Brain Trust?

Legal Definition
Brain trust began as a term for a group of close advisers to a political candidate or incumbent, prized for their expertise in particular fields. The term is most associated with the group of advisers to Franklin Roosevelt during his presidential administration. More recently the use of the term has expanded to encompass any group of advisers to a decision maker, whether or not in politics.
-- Wikipedia
Legal Definition
A group of experts appointed to advise a government or politician.