A clutch is a mechanical device which engages and disengages power transmission especially from driving shaft to driven shaft.
In the simplest application, clutches connect and disconnect two rotating shafts in these devices, one shaft is typically attached to an engine or other power unit while the other shaft provides output power for work. While typically the motions involved are rotary, linear clutches are also possible.
In a torque-controlled drill for instance, one shaft is driven by a motor and the other drives a drill chuck. The clutch connects the two shafts so they may be locked together and spin at the same speed locked together but spinning at different speeds or unlocked and spinning at different speeds.