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