The Collaborative International Dictionary of English v.0.48:

Lunette \Lu*nette"\, n. [F., dim. of lune moon, L. luna. See
   Lune a crescent.]
   1. (Fort.) A fieldwork consisting of two faces, forming a
      salient angle, and two parallel flanks. See Bastion.
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   2. (Far.) A half horseshoe, which lacks the sponge.
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   3. A kind of watch crystal which is more than ordinarily
      flattened in the center; also, a species of convexoconcave
      lens for spectacles.
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   4. A piece of felt to cover the eye of a vicious horse.
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   5. (Arch.) Any surface of semicircular or segmental form;
      especially, the piece of wall between the curves of a
      vault and its springing line.
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   6. An iron shoe at the end of the stock of a gun carriage.
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   Lunette window (Arch.), a window which fills or partly
      fills a lunette.
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WordNet (r) 3.0 (2006):

lunette
    n 1: temporary fortification like a detached bastion
    2: oval or circular opening; to allow light into a dome or vault
       [syn: lunette, fenestella]