What does "caucus" mean?

Caucus: A meeting, especially a preliminary meeting, of persons belonging to a party, to nominate candidates for public office, or to select delegates to a nominating convention, or to confer regarding measures of party policy; a political primary meeting. This day learned that the caucus club meets, at certain times, in the garret of Tom Dawes, the adjutant of the Boston regiment. John Adams's Diary [Feb. , 1763].