Definition
A nut with slots cut across its top face that line up with a hole drilled through the bolt, allowing a cotter pin or safety wire to pass through and lock the nut in place so it cannot rotate loose.
Plain English
A nut with notches across the top, used with a small pin through the bolt so the nut cannot work itself loose from vibration.
Context Anchor
Seen during aircraft maintenance and preflight inspection on parts that must stay securely fastened, such as control linkages, wheel hardware, and other pinned assemblies.
Derivation
From 'castellated,' meaning shaped like the top of a castle wall. The notches around the top of the nut look like the battlements (the up-and-down pattern) on a castle tower. The visual gives you the meaning directly: a nut with castle-like slots cut into it.
Why Pilots Care
Prevents fasteners from backing off under flight vibration, directly affecting airworthiness of safety-critical components.
Intuition Check
Do not read castellated as just decorative. The cut-out slots are there for a locking pin; they are part of how the nut is secured.
Example Sentence 1
The mechanic torqued the castellated nut, then aligned a slot with the cotter pin hole and installed a new cotter pin.
Example Sentence 2
Castellated nuts are used on landing-gear attach bolts to keep them tight after repeated landings.