Continuous collision detection
Checks the path an object sweeps between two frames rather than only its end position, so fast objects stop tunnelling straight through thin walls.
Discrete detection only checks where the object landed after the step. A bullet at speed can sit on one side of a wall one frame and the other side the next, with the wall in between never registered at all — the phenomenon is called tunnelling.
CCD instead sweeps the shape along its motion (swept volumes, conservative advancement) and finds the first contact on the way. It is costly, so engines like Box2D, PhysX and Bullet enable it selectively for exactly those bullets and fast bodies. The alternative — shorter timesteps everywhere — costs more across the whole scene.