Purple state is a term gaining popularity in the US media. It refers to "battleground" or "swing" states in United States Presidential elections. These states are the ones that candidates expend disproportionate resources in. Examples of "purple" states include: Pennsylvania, Ohio, Colorado, Iowa, Wisconsin, and New Hampshire.
"Purple" refers to the tradition of classifying states as "Red" or "Blue", responding to an Electoral College vote of Republican and Democratic presidential candidate, respectively. In this scheme, very close states are colored purple until the final vote tally is finished. In reality, most states are "purple" if viewed on a per-county basis.