Definition
The rotating compass card on a Horizontal Situation Indicator (HSI) or radio magnetic indicator that displays heading in degrees from 0 through 360, with the aircraft's current heading shown at the top index (lubber line). It rotates as the aircraft turns so that the heading under the lubber line always reflects the direction the aircraft is pointing.
Plain English
The round, numbered dial on the HSI that shows which direction the aircraft is pointing. As you turn, the dial turns with you, and the number at the top is your current heading.
Context Anchor
Seen on the face of an HSI or heading indicator during instrument flying, especially when checking heading or turning to a new heading.
Derivation
Azimuth comes from the Arabic 'as-sumut,' meaning 'the directions.' In navigation, azimuth refers to a horizontal direction measured in degrees around a circle. The card is called an azimuth card because it displays those directional degrees.
Why Pilots Care
It gives an immediate visual picture of heading and course deviation without requiring the pilot to interpret separate instruments.
Intuition Check
Card does not mean a paper card here. It means the circular direction scale on the instrument.
Example Sentence 1
As the aircraft turned right, the azimuth card rotated counterclockwise until 270 sat under the lubber line.
Example Sentence 2
With the azimuth card aligned, the pilot could see the course deviation needle pointing left of the selected radial.