Invincibility frames
A brief window after taking a hit — or during a dodge — when the character cannot be hurt again, measured in frames.
Without them, a player who touches an enemy would lose all their health in a fraction of a second, because the collision repeats every frame for as long as the bodies overlap. Invincibility frames grant a short immunity after a hit so each injury costs a sensible amount, and they are almost always signalled by making the character flash.
As a design tool they graduated from stopgap to core mechanic: Dark Souls' dodge roll grants i-frames mid-move, so a well-timed roll passes straight through an attack. The frame count then becomes a finely tuned balance parameter, close kin to frame data and hitstop.