A chuck is a specialized type of clamp. It is used to hold an object with radial symmetry, especially a cylinder. In drills and mills it is used to hold the rotating tool whereas in lathes it holds the rotating work piece. On a lathe the chuck is mounted on a spindle which rotates with in the head stock. For some purposes an additional chuck may be mounted on the non rotating tailstock.
Many chucks have jaws, which are arranged in a symmetrical pattern like the points of star. These jaws are tightening up to hold the tool or work piece. Often the jaws will be tightened up or loosened with the help of a chuck key, which is a wrench like tool made purpose. Many jawed chucks however are of keyless variety, and their tightening and loosing is by hand force alone.