From the Oxford Dictionary
Early 19th century: from the name of Governor Elbridge Gerry of Massachusetts + salamander, from the supposed similarity between a salamander and the shape of a new voting district on a map drawn when he was in office (1812), the creation of which was felt to favour his party; the map (with claws, wings, and fangs added) was published in the Boston Weekly Messenger, with the title The Gerry-Mander.
More on gerrymandering from Chris Ingraham at Washington Post: This is the best explanation of gerrymandering you will ever see and This is actually what America would look like without gerrymandering
The Cook Partisan Voting Index, also called PVI, is a measurement of how strongly a United States congressional district or state leans toward the Democratic or Republican Party, compared to the nation as a whole. The Cook Political Report introduced the PVI in August 1997 to better gauge the competitiveness of each district using the 1992 and 1996 presidential elections as a baseline. The index is based on analysis by the Center for Voting and Democracy (now FairVote) for its July 1997 Monopoly Politics report.