A seal in washer

A washer is a thin plate (typically disk-shaped) with a hole (typically in the middle) that is normally used to distribute the load of a threaded fastener. Other uses are as a spacer, spring (wave washer), wear pad, washer seal, preload indicating device, locking device, and to reduce vibration (rubber washer).

Washers are usually metal or plastic. High quality capscrews require hardened steel washers to prevent the loss of pre-load due to brinelling after the torque is applied.

And a washer seal made of rubber, plastic or other elastic material, used to prevent liquid leaks through a connection.