cas·tel·la·tion \ˌkastəˈlāshən\noun (-s) Etymology: Medieval Latin castellation-, castellatio, from castellatus + -ion, -io -ion 1.: the act of castellating 2.: a castellated structure 3. a.: battlement b.: a groove or recess in a castellated structure (as a nut) < a cotter pin passing through the castellation and the hole in the bolt >