The term gerrymander refers to the deliberate tailoring of electoral district boundaries to favor the political party in power and decrease the representation of their opponents. The party out of power is generally confined to a few highly concentrated districts, while the dominant power attempts to include majorities of their supporters in multiple districts.
Congressional and state legislative districts are usually drawn by state assemblies in response to new census data.
The practice of partisan boundary manipulation dates to the country's early history. The name is derived from Elbridge Gerry, a signer of the Declaration of Independence and vice president under James Madison. While serving as governor of Massachusetts in 1811, he oversaw the redistricting of the Commonwealth to favor the Jeffersonian Republicans. The reformulation of the Essex district was so egregious that it was said to resemble a salamander with a long thin trunk and protrusions at the ends. Somebody commented that it was not a salamander, but a "Gerrymander." Gerry’s name thus became attached to the process, but he was clearly not the originator.
The practice has never entirely disappeared, although state constitutions usually have some rule controlling the demarcation of districts. Supreme Court rulings on reapportionment have also curtailed the freedom of state legislatures to gerrymander.
Elbridge Gerry pronounced his last name with a hard G (as in Gary), but the term gerrymander is usually pronounced with a soft G (and is sometimes spelled jerrymander).