The Collaborative International Dictionary of English v.0.48:

Fortification \For`ti*fi*ca"tion\, n. [L. fortificatio : cf. F.
   fortification.]
   1. The act of fortifying; the art or science of fortifying
      places in order to defend them against an enemy.
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   2. That which fortifies; especially, a work or works erected
      to defend a place against attack; a fortified place; a
      fortress; a fort; a castle.
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   Fortification agate, Scotch pebble.

   Syn: Fortress; citadel; bulwark. See Fortress.
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