packing and cracking
n. Techniques used to redraw electoral boundaries to favor one political party over another.
Throughout Florida packing and cracking have helped the Republicans to gain an eleven-seat majority in the state's congressional delegation, even though Florida's voting population—as the 2000 election made famously clear—is evenly divided between Republicans and Democrats.
—Don Peck & Casey Caitlin, “Packing, cracking, and kidnapping: the science of gerrymandering,” The Atlantic Monthly, January 01, 2004
DeLay has perverted the redistricting process in three ways: By targeting Democratic incumbents through illegal packing and cracking of districts; by conducting a mid-decade redistricting without waiting for the decennial census; by stacking the deck in favor of Republicans in order to influence the presidential election.
—James O. Goldsborough, “Redistricting and reapportionment,” The San Diego Union-Tribune, December 22, 2003
1983 (earliest)
But Rom said there is another issue in the suit that Caffrey has not addressed yet, which he described as "packing and cracking."

He argued that, aside from the numbers, the rejected districts were faulty because they had the effect of "packing" almost all of Boston's black voters into two districts and "cracking" the Hispanic vote among six districts. The result, he said, would be that blacks could never elect more than two candidates, while Hispanic voters would likely be shut out altogether.
—Christopher B. Daly, “Boston Faces Gerrymandering Mess 172 Years After The Original,” The Associated Press, July 30, 1983
The unfair redrawing of electoral boundaries has long been known as gerrymandering (1812). As weird as it may sound, it was the combination of the last name of Elbridge Gerry, a former governor of Massachusetts, and salamander ("a small lizard-like amphibian") that gave us the word gerrymander.

This strange beast was formed when governor Gerry reshaped his state's voting districts to favor his party. One such district looked suspiciously like a salamander, and was drawn as such in an editorial cartoon by Gilbert Stuart. His editor immediately dubbed the creature a "Gerry-mander," and the name stuck. (Some trivia: Stuart was the same man who painted the portrait of George Washington that appears on the U.S. $1 bill.)

Legitimate forms of electoral redrawing are called either redistricting or reapportionment. In the U.S., the Supreme Court has ordered that districts be reapportioned after each census (every 10 years) to allow for population shifts. However, there's nothing legitimate about packing and cracking:

Packing means redrawing a boundary so that more of a party's supporters are crammed into a district that is already heavily slanted towards that party. This is a bad thing because it means those extra votes are "wasted" since they go to a candidate who would win in a landslide anyway.

Cracking means taking a constituency that typically favors a particular party (for example, African-American voters, who typically vote Democrat in the U.S.) out of a single district and splitting them among a number of different districts. This is bad because it prevents that constituency from voting en masse to elect their preferred party.

Some commentators have suggested that packing and cracking and similar techniques are at least partly to blame for the fact that incumbent election losses are becoming increasingly rare, since most electoral districts are now populated with solid majorities of supporters of one party or the other.