A circlip (a portmanteau of ‘circle’ and ‘clip’), also known as a C-Clip, Seeger, snap ring or Jesus clip, is a type of fastener or retaining ring consisting of a semi-flexible metal ring with open ends which can be snapped into place, into a machined groove on a dowel pin or other part to permit rotation but to prevent lateral movement. There are two basic types: internal and external, referring to whether they are fitted into a bore or over a shaft. Circlips are often used to secure pinned connections.

Common examples include “E-clips” (e-ring) and the (both internal and external) snap ring (US) or circlip (UK). This general type of fasteners are sized to provide an interference fit onto (or into, in the case of an internal fastener) a groove or land when in use, such that they must be elastically deformed in order to install or remove them.

Since circlips are stamped out of sheetmetal there is a smooth side and a rough side. To prevent potential damage, circlips are installed with the smooth side facing the part and the rough side facing out. Wet or dry lubrication is required for the circlip to maintain its function.

Circlips are designed to be removed with special circlip pliers (snap ring tool US) which can be reassembled for internal or external clips, but in field expedient situations, a pair of needle-nose pliers (for internal clips) or leverage with a flat-headed screwdriver (internal or external) are sometimes used.

Source Wikipedia