A collet is a holding device—specifically, a subtype of chuck

Chuck (engineering)

A chuck is a specialized type of clamp used to hold an object, usually an object with radial symmetry, especially a cylindrical object. It is most commonly used to hold a rotating tool or a rotating workpiece...

—that forms a collar

Collar

Collar may refer to:Human neckwear:*Collar , the part of a garment that fastens around or frames the neck*Ruff collar*Slave collar*Collar , a device of any material placed around the neck of the submissive partner in BDSM...

 around the object to be held and exerts a strong clamping force on the object when it is tightened, usually via a tapered outer collar. It may be used to hold a workpiece or a tool.

A collet is a sleeve with a (normally) cylindrical

Cylinder (geometry)

A cylinder is one of the most basic curvilinear geometric shapes, the surface formed by the points at a fixed distance from a given line segment, the axis of the cylinder. The solid enclosed by this surface and by two planes perpendicular to the axis is also called a cylinder...

 inner surface and a conical

Cone (geometry)

A cone is an n-dimensional geometric shape that tapers smoothly from a base to a point called the apex or vertex. Formally, it is the solid figure formed by the locus of all straight line segments that join the apex to the base...

 outer surface. The collet can be squeezed against a matching taper such that its inner surface contracts to a slightly smaller diameter, squeezing the tool or workpiece whose secure holding is desired.