Gerrymandering is the manipulation of district boundaries to give an advantage to one particular group (political, racial, religious, ethnic, urban, rural, etc.), often in an attempt to keep incumbent elected officials in power.
The name originated in the early 19th century, when Massachusetts Gov. Elbridge Gerry signed a law creating a district that benefited his party. It was shaped like a salamander, hence "Gerrymander."
The civil rights movement and the 1965 Voting Rights Act helped end (or at least reduce) the practice as it had been applied to race. But Republicans and Democrats still squabble over boundaries when it comes time to redistrict.