Nash equilibrium
The unexploitable strategy for a spot: the one no opponent can profit from by deviating away from, regardless of how they adjust.
Nash equilibrium is the correct starting point for a spot like push/fold, not the final answer for every opponent. Against a player who deviates from it, by folding too much or calling too much, you can profitably deviate too, jamming even wider against an over-folder or tightening against an over-caller. The equilibrium is the baseline everyone eventually gets punished for drifting too far from, not a fixed script to play regardless of the table.