- A pattern along the top of a parapet (fortified wall), most often in the form of multiple, regular, rectangular spaces in the top of the wall, through which arrows or other weaponry may be shot, especially as used in medieval European architecture.
- The act of crenellating; adding a top row that looks like the top of a medieval castle.
Ultimately from Latin crenella as a diminutive of crena (literally “a notch or serration”). The word also appears in Old French as cren (“a notch”) or crener (“to notch”). The word shares its origins with the common English word cranny, similarly meaning “a small opening, as in a wall or rock face; a crevice”.