The strike where the most options expire worthless — where price is often drawn into expiration.
Max pain is the strike price at which the largest dollar amount of options — calls and puts combined — expires worthless, i.e. where option buyers feel the most "pain." Because dealer hedging around heavy open interest tends to gravitate toward that level into expiration, price often drifts toward max pain as the clock runs out. It is a magnet, not a guarantee.
More terms: Gamma wall · Delta neutral · Gamma · 0DTE options · Dealer hedging · Call wall