In the United States, a Political Action Committee, or PAC, is the name commonly given to a private group, regardless of size, organized to elect political candidates or to advance the outcome of a political issue or legislation. Legally, what constitutes a "PAC" for purposes of regulation is a matter of state and federal law. Under the Federal Election Campaign Act, an organization becomes a "political committee" by receiving contributions or making expenditures in excess of $1,000 for the purpose of influencing a federal election.

Source: Wikipedia

For more information about PAC please see: http://en.wikipedia.org/wiki/Political_action_committee