The Collaborative International Dictionary of English v.0.48:

Crenel \Cre*nel"\ (kr?-n?l"), n.
   See Crenelle.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Crenelle \Cre*nelle"\, Crenel \Cre*nel"\ (kr?-n?l"), n. [OF.
   crenel, F. cr?neau, LL. crenellus, kernellus, dim. (prob.)
   fr. L. crena notch. See Crenny.]
   1. An embrasure or indentation in a battlement; a loophole in
      a fortress; an indentation; a notch. See Merlon, and
      Illust. of Battlement.
      [1913 Webster]

   2. (Bot.) Same as Crenature.
      [1913 Webster]

WordNet (r) 3.0 (2006):

crenel
    n 1: one of a series of rounded projections (or the notches
         between them) formed by curves along an edge (as the edge
         of a leaf or piece of cloth or the margin of a shell or a
         shriveled red blood cell observed in a hypertonic solution
         etc.) [syn: scallop, crenation, crenature, crenel,
         crenelle]
    2: a notch or open space between two merlons in a crenelated
       battlement [syn: crenel, crenelle]
    v 1: supply with battlements [syn: crenel, crenelate,
         crenellate]