Plain English Meaning
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.
Full 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.
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
comes from the Arabic 'as-sumut,' meaning 'the directions.' In , azimuth refers to a horizontal direction measured in degrees around a . The is called an azimuth card because it displays those directional degrees.