The tipping point state (or district) is the state/district that tips the winning candidate over the edge (gives them an Electoral College majority) when you give them the Electoral votes from each state/district they won, starting with the largest margin by percentage, and going down through decreasing margins. In other words, it is the state that, if you uniformly shifted the margins in all states by the same amount, would be required for the winner to win to still have an Electoral College majority. 

In 2020, the tipping point state was Wisconsin. Note that the tipping point state is the one required to give the winner a majority of Electoral votes: Without Wisconsin or the states that were closer than it, Biden would have won exactly 269 votes, so Wisconsin was the state that tipped him into the majority. This also means that, in the unlikely event of an Electoral College tie, the tipping point state will actually be a state not won by the winner.