The Collaborative International Dictionary of English v.0.48:

Thimble \Thim"ble\, n. [OE. thimbil, AS. ??mel, fr. ??ma a
   thumb. [root]56. See Thumb.]
   1. A kind of cap or cover, or sometimes a broad ring, for the
      end of the finger, used in sewing to protect the finger
      when pushing the needle through the material. It is
      usually made of metal, and has upon the outer surface
      numerous small pits to catch the head of the needle.
      [1913 Webster]

   2. (Mech.) Any thimble-shaped appendage or fixure.
      Specifically:
      (a) A tubular piece, generally a strut, through which a
          bolt or pin passes.
      (b) A fixed or movable ring, tube, or lining placed in a
          hole.
      (c) A tubular cone for expanding a flue; -- called
          ferrule in England.
          [1913 Webster]

   3. (Naut.) A ring of thin metal formed with a grooved
      circumference so as to fit within an eye-spice, or the
      like, and protect it from chafing.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Ferrule \Fer"rule\ (f[e^]r"r[i^]l or f[e^]r"r[.u]l; 277), n.
   [Formerly verrel, F. virole, fr. L. viriola little bracelet,
   dim. of viriae, pl., bracelets; prob. akin to viere to twist,
   weave, and E. withe. The spelling with f is due to confusion
   with L. ferrum iron.] [Sometimes spelled ferule.]
   1. A ring or cap of metal put round a cane, tool, handle, or
      other similar object, to strengthen it, or prevent
      splitting and wearing.
      [1913 Webster]

   2. (Machinery) Any of various circular or cylindrical metal
      objects used at joints in a tube, pipe, or rod, especially
      to assist making a tight seal at a joint.
      [PJC]

   3. Hence: (Steam Boilers) A bushing for expanding the end of
      a flue to fasten it tightly in the tube plate, or for
      partly filling up its mouth.
      [1913 Webster]

   4. Hence: (Chemistry) A bushing used at the joints of metal
      tubing in HPLC equipment to make a tight seal.
      [PJC]

   5. (Fishing) One of several small rings at the top of a
      casting rod which holds the fishing line. --RHUD
      [PJC]

WordNet (r) 3.0 (2006):

ferrule
    n 1: a metal cap or band placed on a wooden pole to prevent
         splitting [syn: ferrule, collet]