crenel (n.)

also crenelle, "open space on an embattlement," mid-13c., carnel, kernel, from Old French crenel "a notch; embrasure" (12c., Modern French créneau), apparently a diminutive of cren "a notch," from Latin crena (see cranny).

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Definitions of crenel from WordNet
crenel (n.)
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.);
Synonyms: scallop / crenation / crenature / crenelle
crenel (n.)
a notch or open space between two merlons in a crenelated battlement;
Synonyms: crenelle
crenel (v.)
supply with battlements;
Synonyms: crenelate / crenellate