The word caucus probably derives from an Algonquin word caucasu, which refers to a (usually circular) meeting of chiefs of tribes, where no one person dominated. (See the Webster's 1913 Edition dictionary. The Oxford English Dictionary has more information, but it's not readily available on-line.)
Why Do You Call It "Caucus"?
There is, of course, the use of "caucus" in American politics as a group-decision process (although that may have unfortunate associations for some folks!). In fact, there was, in the long-ago days, an on-line "Dukakis Caucus" host that used the text-based version of Caucus.