According to Hans Kmoch's definition in Pawn Power in Chess (1956), the distance between the back rank and a pawn

A move or way of playing that proves the opponent's move is an error. Particularly refers to strong responses that show the opponent's strategy or combination doesn't work. Includes not only tactical mistakes but also moves that prove opening novelties are unsound

A positional move that prevents the opponent's plans beforehand (e.g., prophylaxis)

Reaching the same position through different move orders. Example: 1.d4 Nf6 2.c4 e6 3.Nf3 d5 = 1.d4 d5 2.Nf3 Nf6 3.c4 e6

A structure that reproduces an opening with colors reversed. For example, the Reverse Sicilian has White and Black's roles reversed while maintaining the same structure