It's only use in connection with political parties. It means a planned or spontaneous working meeting, of a group within the group. The meeting is usually to make decisions about policy or choice of candidates. It is basically a noun, but in English it is common to use nouns informally as verbs. Dictionaries include the verb usage when it becomes frequent. Here are some actual examples I found using Google--lightly edited, in some cases. Noun: "Two members of Congress have announced the formation of the Congressional STEAM Caucus. The group will host briefings and advocate for policy changes that will encourage educators to integrate arts, broadly defined, with traditional Science, Technology, Engineering and Math curriculum." "'We want to show people that Muslims are a part of the American fabric,' said Salam, a Weston resident who is president of the American Muslim Democratic Caucus of Florida. The 2016 elections will be the first her group has been an officially recognized caucus of the Florida Democratic Party." "The Iowa caucuses are an electoral event in which residents of the U.S. state of Iowa meet in precinct caucuses in all of Iowa's 1,682 precincts and elect delegates to the corresponding county conventions." Verb: (this example is from 1914) "We then sent them our bill. They beat it. How did they beat it? I will tell you how they beat our Direct Primary Bill. First the Democrats caucused against it. Then the Republicans caucused against it. The two great political parties caucused to defeat this bill of the people."

Dan's answer is excellent. I'd just add that "caucus" can also be an adjective, such as in "caucus leader" or "caucus organizer"