A vote for candidates of different political parties on the same ballot, instead of for candidates of only one party. In the presidential elections, for example, a voter may choose a Republican candidate for president, but a Democratic candidate for senator. Split-ticket voting is not allowed in primaries (see closed primary, direct primary, open primary). The increasing occurrence of split-ticket voting reflects support of individual candidates rather than unswerving party loyalty.
A ballot cast for candidates of more than one party, as in I'm registered as an Independent, and indeed I usually vote a split ticket. This idiom uses ticket in the sense of "a list of nominees for office," a usage dating from the late 1700s. Also see straight ticket.