What does "calipers" mean?

Calipers: An instrument, usually resembling a pair of dividers or compasses with curved legs, for measuring the diameter or thickness of bodies, as of work shaped in a lathe or planer, timber, masts, shot, etc.; or the bore of firearms, tubes, etc.; -- called also caliper compasses, or caliber compasses. Caliper square, a draughtsman's or mechanic's square, having a graduated bar and adjustable jaw or jaws. Knight. -- Vernier calipers. See Vernier.