The Collaborative International Dictionary of English v.0.48:

Speculum \Spec"u*lum\, n.; pl. L. Specula, E. Speculum. [L.,
   fr. specere to look, behold. See Spy.]
   1. A mirror, or looking-glass; especially, a metal mirror, as
      in Greek and Roman archaeology.
      [1913 Webster]

   2. A reflector of polished metal, especially one used in
      reflecting telescopes. See Speculum metal, below.
      [1913 Webster]

   3. (Surg.) An instrument for dilating certain passages of the
      body, and throwing light within them, thus facilitating
      examination or surgical operations.
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   4. (Zool.) A bright and lustrous patch of color found on the
      wings of ducks and some other birds. It is usually
      situated on the distal portions of the secondary quills,
      and is much more brilliant in the adult male than in the
      female.
      [1913 Webster]

   Speculum metal, a hard, brittle alloy used for making the
      reflectors of telescopes and other instruments, usually
      consisting of copper and tin in various proportions, one
      of the best being that in which there are 126.4 parts of
      copper to 58.9 parts of tin, with sometimes a small
      proportion of arsenic, antimony, or zinc added to improve
      the whiteness.
      [1913 Webster]

WordNet (r) 3.0 (2006):

speculum
    n 1: a mirror (especially one made of polished metal) for use in
         an optical instrument
    2: a medical instrument for dilating a bodily passage or cavity
       in order to examine the interior