The tipping point state is defined as the state that has the decisive Electoral College vote when all states are organized by margin. That is, if we order states from reddest to bluest or bluest to reddest, the one with the 270th vote is the tipping point state. Note that by this definition, it is possible for two states to be the tipping point state if one of them gives the 270th vote when ordered from reddest to bluest and a different one gives the 270th vote when the order is reversed. This is because there are an even number of total votes, so this situation occurs when one state has the 269th vote but not the 270th (so that a candidate winning just that state and not the next one would result in a tie rather than a victory). In this situation, it is said that the bluer of the two states (the one that has the 270th vote when ordered from reddest to bluest) is the tipping point state for a Republican victory, and the redder one is the tipping point state for a Democratic victory. This is what happened in 2020, with PA being the tipping point state for a Republican victory and WI being the tipping point for a democratic victory.

Also note that Washington D.C. and Maine and Nebraska's Congressional districts are treated as their own states for this definition, since they have their own electoral votes.

The blueness of a state is defined as Democratic vote percentage minus the Republican vote percentage in the POTUS election. All states that are bluer than the tipping point state resolve YES, and all those that are redder (i.e., less blue) resolve NO. If one state is the sole tipping point state, then it resolves 50%, but if there are two tipping point states, the redder one resolves NO and the bluer one resolves YES.

See also: /PlasmaBallin/what-will-be-the-tipping-point-stat