In politics, a partisan is a committed member of a political party. In multi-party systems, the term is used for politicians who strongly support their party's policies and are reluctant to compromise with their political opponents.
In the United States, the meaning of the term has changed dramatically over the last 60 years. Before the American National Election Study (described in Angus Campbell et al., in The American Voter) began in 1952, an individual's partisan tendencies were typically determined from their voting behavior. Since then, "partisan" has come to refer to an individual with a psychological identification with one or the other of the major parties.
The dictionary definition of partisan at Wiktionary
Graphic showing Rise of Partisanship in the US House of Representatives 1949-2011