Nash Equilibrium

If two strategies are best responses to each other, then they are in Nash equilibrium.

In general, we say that two strategies $s_1$ and $s_2$ are in Nash equilibrium (NE) if:

1. under the assumption that agent $i$ plays $s_1$, agent $j$ can do no better than play $s_2$; and
2. under the assumption that agent $j$ plays $s_2$, agent $i$ can do no better than play $s_1$

Neither agent has any incentive to deviate from a NE.