The Collaborative International Dictionary of English v.0.48:

Battlement \Bat"tle*ment\ (-ment), n. [OE. batelment; cf. OF.
   bataillement combat, fr. batailler, also OF. bastillier,
   bateillier, to fortify. Cf. Battle, n., Bastile,
   Bastion.] (Arch.)
   (a) One of the solid upright parts of a parapet in ancient
       fortifications.
   (b) pl. The whole parapet, consisting of alternate solids and
       open spaces. At first purely a military feature,
       afterwards copied on a smaller scale with decorative
       features, as for churches.
       [1913 Webster]

WordNet (r) 3.0 (2006):

battlement
    n 1: a rampart built around the top of a castle with regular
         gaps for firing arrows or guns [syn: battlement,
         crenelation, crenellation]