Definition
A cockpit instrument display, usually a vertical needle, that shows the aircraft's lateral position relative to a selected navigation course. When the needle is centered, the aircraft is on course. When the needle deflects left or right, it indicates the course is to that side, telling the pilot which way to steer to return to the desired track. The CDI is driven by VOR, localizer, GPS, or RNAV signals.
Plain English
An instrument needle that tells you whether you are on the line you wanted to fly. If the needle is in the middle, you are on the line. If it leans left, the line is to your left and you need to steer left to get back on it.
Context Anchor
Seen on navigation instruments and displays when using VOR, localizer, or GPS-based course guidance.
Derivation
Course' means the path you intend to fly. 'Deviation' comes from Latin 'deviare', meaning to wander off the way. 'Indicator' simply means something that shows. So the name itself describes the job: it shows how far you have wandered off your intended path.
Why Pilots Care
Precise course tracking prevents navigation errors and keeps the aircraft aligned during instrument approaches.
Intuition Check
A CDI is not a heading indicator. It does not show where the nose is pointed; it shows where the aircraft is compared with the selected course.
Example Sentence 1
The pilot watched the CDI center as the aircraft intercepted the inbound radial.
Example Sentence 2
Keeping the CDI centered throughout the ILS approach maintained alignment with the runway centerline.