rea chain link made in two parts, one piece fitting like a collar below the bolt head of the other and turning freely about it -- but this is exactly how i would define a swivel, (as a noun)

and swively hips.. still has a very definate sexual connotation!
i have hooks with swivels, and the hooks end in a collar, and a bolt like pieces passes through, and the bolt then is looped, to start a chain.

swivel chairs are just the same thing! a collar, and the bolt that passes through becomes the legs, the collar supports the seat.. (and now days there are ball or roller bearing between the collar and the bolt so things swivel readily.