They are the (purple) states that can be won by either Republicans or Democrats during presidential elections. Think Pennsylvania or Michigan, there was a ton of dramatic campaigning in those states last year, including an attempted assassination.

In contrast there are reliably Democrat (blue) states like California and Washington, and reliably Republican (red) states like Texas and West Virginia

It's another very US-centric concept

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twinsies! Really was surprised when that ended up being the purple category. Although maybe they pretty much had to do it as a joke