A chain was a series of linked metal loops used for various purposes.
When Sub-commander T'Pol suggested to use stun grenades, detonate them, and use both shuttlepods to bring the Novans back to Enterprise NX-01, Captain Jonathan Archer asked her what to do then with the Novans – put them in chains? T'Pol then stated that Mister Reed would devise appropriate restraints. This suggestion was dismissed by Archer as Enterprise was no slave ship. (ENT: "Terra Nova")
In 2266, Lenore Karidian compared the Karidian Company of Players' schedule to a chain, saying that if one link broke, the rest of it would collapse. (TOS: "The Conscience of the King")
During his visit to the Shore Leave Planet, Esteban Rodriguez's imagination gave form to a chained Bengal tiger. (TOS: "Shore Leave")
In 2367, a hologram of the ghost of Jacob Marley showed off the ghostly chains he wore. (TNG: "Devil's Due")
Risik planned to get "a big tattoo of a rusty chain, that goes from my leg and wraps around up my neck", but one of his crewmates said he had to earn it first. Risik then said he would when he took over a ship. (LD: "Something Borrowed, Something Green")