Nash Equilibrium
The point where nobody can improve by changing their mind alone.
What it actually is
A Nash equilibrium is a set of choices where no single player can do better by switching, assuming everyone else holds still. That is the whole test: freeze the others, ask yourself if you would deviate, and if the answer is no for every player at once, you have found the resting point. Notice what the definition does not promise. It says nothing about the outcome being good, fair, or efficient, only that it is stable, a place nobody can escape on their own. Most real games have exactly one obvious one, some have several, and the trap is that you can be stuck in a bad equilibrium precisely because moving first only makes it worse for you.
Where you'll see it
Look at two gas stations across the street from each other posting nearly identical prices. Neither one raises a cent, because the moment it does, every driver crosses to the cheaper pump and it loses the block. Neither one drops a cent either, because that just torches its own margin while the other happily matches a beat later. So they sit at the same low price, both wishing they could charge more, both unable to move alone. That standoff is a Nash equilibrium, and it is why John Nash won the 1994 economics Nobel for describing it.
Spot it in yourself
Whenever you catch yourself saying "this is stupid, but if I change and nobody else does, I just lose," you are staring at an equilibrium. Stable does not mean smart, it means nobody wants to be the sucker who blinks first.
The bias underneath
An equilibrium is the state everyone stays glued to because moving first only hurts, which is exactly the inertia status quo bias runs on.