Definition
The course index is the fixed triangular marker at the top (12 o'clock position) of the VOR course deviation indicator's compass card. It points to the radial or course currently selected by the Omnibearing Selector (OBS), and the number aligned beneath it is the course the aircraft is set to track to or from the VOR station.
Plain English
It's the little fixed pointer at the top of the VOR dial. Whatever number sits under that pointer is the course you've chosen to fly.
Context Anchor
Seen on a VOR indicator when using the OBS knob to choose the course you want to fly.
Derivation
Index' comes from the Latin 'index,' meaning 'pointer' or 'forefinger' -- the same root as a finger that points something out. The course index literally points out which course you've selected.
Why Pilots Care
It lets the pilot select a precise radial so the course deviation indicator can show whether the aircraft is on, left of, or right of the desired track.
Intuition Check
Do not read course index as a school course or a book index. In this instrument context, it means the fixed mark that points to the selected VOR course.
Example Sentence 1
She turned the OBS until 270 was under the course index, then flew the needle to intercept the radial.
Example Sentence 2
With the course index aligned to the final approach course, the needle centered as the aircraft intercepted the radial.