Source : Webster's Revised Unabridged Dictionary (1913)

Crenelation \Cren`el*a"tion\ (-?"sh?n), n.
   The act of crenelating, or the state of being crenelated; an
   indentation or an embrasure. [Written also {crenellation}.]

Source : WordNet®

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