Plain English Meaning
A single instrument that shows where the airplane is pointing and where it is in relation to the course you want to fly, so you can see the whole navigation picture at a glance.
Full Definition
A cockpit instrument that combines a heading indicator and a course deviation display into a single pictorial presentation, showing the aircraft's heading, the selected course, and the aircraft's position relative to that course on one face. It is the display element of a remote indicating compass system and is sometimes referred to as a horizontal situation indicator (HSI).
Context Anchor
Seen in and remote indicating compass discussions, especially when describing displays such as a horizontal situation indicator.