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.
      [1913 Webster]

   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.
      [1913 Webster]

   Fortification agate, Scotch pebble.

   Syn: Fortress; citadel; bulwark. See Fortress.
        [1913 Webster]

WordNet (r) 3.0 (2006):

fortification
    n 1: defensive structure consisting of walls or mounds built
         around a stronghold to strengthen it [syn: fortification,
         munition]
    2: the art or science of strengthening defenses
    3: the addition of an ingredient for the purpose of enrichment
       (as the addition of alcohol to wine or the addition of
       vitamins to food)