Source : Webster's Revised Unabridged Dictionary (1913)

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

   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.

Source : WordNet®

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