Padlock
From wipipedia.org
A padlock is a device for locking things together. It may be used for securing metal and leather bondage equipment to make it escape-proof.
If several padlocks are used in a bondage session, it is advisable to get a set that all open with the same key, so that the padlocks can be removed rapidly without excessive fumbling for keys. Alternatively, each key and padlock should be labelled.