Definition
The four principal points of the compass: north (N), east (E), south (S), and west (W), spaced 90 degrees apart and used as the primary reference points for direction.
Plain English
The four main directions on a compass: north, east, south, and west.
Context Anchor
Seen on the magnetic compass, heading indicator, and aviation charts when a pilot is orienting the aircraft or reading direction.
Derivation
From Latin cardinalis, meaning 'principal' or 'on which something turns' (from cardo, 'hinge'). These four directions are the 'hinges' all other directions turn around — the points everything else is measured from.
Why Pilots Care
All headings, courses, and bearings are measured relative to these four points. On the compass card, N, E, S, and W are the anchor markings; the numbered headings between them (030, 060, 120, etc.) only make sense in reference to them.
Intuition Check
Do not read cardinal directions as every possible direction on the compass. In this context, cardinal directions means only the four main ones: north, east, south, and west.
Example Sentence 1
After the turn, the heading indicator settled on a cardinal direction — due east, 090.
Example Sentence 2
During the preflight, the mechanic verified deviation at each of the four cardinal directions.