The Collaborative International Dictionary of English v.0.48:

Pulley \Pul"ley\, n.; pl. Pulleys. [F. poulie, perhaps of
   Teutonic origin (cf. Poll, v. t.); but cf. OE. poleine,
   polive, pulley, LL. polanus, and F. poulain, properly, a
   colt, fr. L. pullus young animal, foal (cf. Pullet,
   Foal). For the change of sense, cf. F. poutre beam,
   originally, a filly, and E. easel.] (Mach.)
   A wheel with a broad rim, or grooved rim, for transmitting
   power from, or imparting power to, the different parts of
   machinery, or for changing the direction of motion, by means
   of a belt, cord, rope, or chain.
   [1913 Webster]

   Note: The pulley, as one of the mechanical powers, consists,
         in its simplest form, of a grooved wheel, called a
         sheave, turning within a movable frame or block, by
         means of a cord or rope attached at one end to a fixed
         point. The force, acting on the free end of the rope,
         is thus doubled, but can move the load through only
         half the space traversed by itself. The rope may also
         pass over a sheave in another block that is fixed. The
         end of the rope may be fastened to the movable block,
         instead of a fixed point, with an additional gain of
         power, and using either one or two sheaves in the fixed
         block. Other sheaves may be added, and the power
         multiplied accordingly. Such an apparatus is called by
         workmen a block and tackle, or a fall and tackle.
         See Block. A single fixed pulley gives no increase of
         power, but serves simply for changing the direction of
         motion.
         [1913 Webster]

   Band pulley, or Belt pulley, a pulley with a broad face
      for transmitting power between revolving shafts by means
      of a belt, or for guiding a belt.

   Cone pulley. See Cone pulley.

   Conical pulley, one of a pair of belt pulleys, each in the
      shape of a truncated cone, for varying velocities.

   Fast pulley, a pulley firmly attached upon a shaft.

   Loose pulley, a pulley loose on a shaft, to interrupt the
      transmission of motion in machinery. See Fast and loose
      pulleys, under Fast.

   Parting pulley, a belt pulley made in semicircular halves,
      which can be bolted together, to facilitate application
      to, or removal from, a shaft.

   Pulley block. Same as Block, n. 6.

   Pulley stile (Arch.), the upright of the window frame into
      which a pulley is fixed and along which the sash slides.


   Split pulley, a parting pulley.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pulley block
    n 1: a simple machine consisting of a wheel with a groove in
         which a rope can run to change the direction or point of
         application of a force applied to the rope [syn: pulley,
         pulley-block, pulley block, block]