Master link

A master link or quick-release link is a roller chain accessory that allows convenient connection and disconnection of a chain without the need for a chain tool. It acts as a set of the chain`s outer plates, so joining two sets of the chain`s inner plate ends. Such master links may or may not be re-usable. A chain tool is nonetheless needed to a.....
Found on http://en.wikipedia.org/wiki/Master_link

master link

a pin link that has one removable side plate located by a clip, or is staked (punched) in place. This allows a convenient way of separating chain.
Found on http://www.horizonsunlimited.com/gear-up/motorcycle-terms-and-glossary
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