Look up sheave in Wiktionary, the free dictionary.
Pulley block including sheave.
A sheave/ʃiːv/ is a wheel or roller with a groove along its edge for holding a belt, rope, or cable. When hung between two supports and equipped with a belt, rope or cable, one or more sheaves make up a pulley. The words sheave and pulley are sometimes used interchangeably.
A sheave can also refer to a pulley which has an adjustable operating diameter for use with a mechanical belt. This is accomplished by constructing the pulley out of several pieces. The two main "halves" of the pulley can be moved closer together or farther apart, thus altering the operational diameter. The usual construction is some sort of locking collar or set screws to secure the components, one half with a threaded central shaft and one half with a threaded center. By rotating the components one can "screw" the parts closer together or further apart, thus changing the distance between the two halves and allowing the belt to ride higher or lower in the groove. The adjustments are constrained to a specific range of size and are not limitless.