Definition
A conical recess cut into the surface of a material around the edge of a drilled hole, sized and angled to allow the head of a flat-head (countersunk) fastener to sit flush with or below the surface. Also refers to the cutting tool used to make this recess.
Plain English
A cone-shaped dip cut around the top of a hole so a screw or rivet head can sit level with the surface instead of sticking up. The tool that makes this dip is also called a countersink.
Context Anchor
Seen in aircraft sheet metal work, structural repair, and maintenance instructions for installing flush screws or rivets.
Derivation
From 'counter-' (against, opposite) and 'sink' (to set below a surface). The word literally describes setting a fastener head down into the material rather than letting it sit on top.
Why Pilots Care
Flush fasteners reduce aerodynamic drag on aircraft skins and prevent snagging. A countersink that is too deep weakens the structure; too shallow and the fastener head protrudes. Both are inspection items on airworthy structure.
Intuition Check
A countersink is not just any drilled hole. It is the angled recess at the top of the hole, sized so a specific fastener head can sit level with the surface.
Example Sentence 1
Before installing the flush rivets, the technician used a countersink to prepare each hole in the wing skin.
Example Sentence 2
After countersinking the access panel, the screws sat level and produced no extra drag.