A series of linked metal rings for fastening or securing something. From the later sense of a jointed measuring line consisting of linked metal rods, chain is also used as a unit of measure for the length of such a measuring line (66 ft).
a chain is no stronger than its weakest link proverbial saying, mid 19th century, often used when identifying a particular point of vulnerability. (In 2000, the popular television game-show The Weakest Link was introduced, turning on the identification and expulsion of the member of a group seen to contribute least; the show's catchphrase is ‘You are the weakest link…goodbye.’)
See also ball and chain.