A grooved pulley or sheave incased in a frame or shell
   which is provided with a hook, eye, or strap, by which it may be
   attached to an object. It is used to change the direction of motion, as
   in raising a heavy object that can not be conveniently reached, and
   also, when two or more such sheaves are compounded, to change the rate
   of motion, or to exert increased force; -- used especially in the
   rigging of ships, and in tackles.