NOUN (2)

1. a small telescope attached to a large telescope to use in setting the line of the larger one;

2. optical device consisting of a tube containing a convex achromatic lens at one end and a slit at the other with the slit at the focus of the lens; light rays leave the slit as a parallel beam;

The Collaborative International Dictionary of English v.0.48:

Collimator \Col"li*ma`tor\, n.
   1. (Astron.) A telescope arranged and used to determine
      errors of collimation, both vertical and horizontal.
      --Nichol.
      [1913 Webster]

   2. (Optics) A tube having a convex lens at one end and at the
      other a small opening or slit which is at the principal
      focus of the lens, used for producing a beam of parallel
      rays; also, a lens so used.
      [1913 Webster]

WordNet (r) 3.0 (2006):

collimator
    n 1: a small telescope attached to a large telescope to use in
         setting the line of the larger one
    2: optical device consisting of a tube containing a convex
       achromatic lens at one end and a slit at the other with the
       slit at the focus of the lens; light rays leave the slit as a
       parallel beam