A retainer ring, also known as a retaining ring or snap ring, is a mechanical fastener used to secure components or assemblies onto a shaft or within a housing or bore. These rings are designed to fit into a groove and act as a shoulder to hold parts in place, preventing them from sliding out of position or becoming disassembled.

Retainer rings come in various types, including internal and external rings, depending on whether they are designed to fit inside a bore or around a shaft, respectively. They can also be categorized by their shapes, such as circlips, E-clips, and spiral rings, each suited for specific applications and load requirements.