A Collet

1. A band, flange, ferrule, or collar, designed to grip and hold a tool or a workpiece under proper control, and usually to release it under control thereafter; such a collet usually is made of a hard, springy material, especially a metal.
2. In jewelry, the rim (of a ring) within which a jewel is set. Compare bezel.
3. (horticulture) In an embryonic plant, the transition zone between the root and the hypocotyl (not clearly distinguishable in most plants.)