Chuck

A chuck is the clamping mechanism that holds a bit or blade in place on a rotating tool -- a drill's chuck grips drill bits, a router's collet does a related job for router bits specifically. Most modern drills use a keyless chuck, tightened by hand; some drill presses and older tools use a keyed chuck, tightened with a small geared key for a firmer, more consistent grip -- worth the extra step when precision or torque really matters.