Definitions of caucus

an informal group of legislators that share an interest in the same policy issues and represents this interest throughout the legislative process and to generally influence the debate on the issue in the public policy arena

The bipartisan Senate India Caucus has an initial 32 members in the 100-strong upper house, including Hillary Clinton.

a meeting, in particular a meeting of people whose goal is political or organizational change

The caucus has a low turnout and is dominated by groups trying to draw attention to local issues such as farming and ethanol subsidies.

a meeting of legislators who are members of the same political party to select candidates or decide policy; the members of such a body

The difference between a caucus and the primaries that most states favor is that to take part in a caucus you have to take part in person.