Definition
An instrument display, usually a vertical needle on a navigation indicator, that shows how far left or right of the selected course the aircraft is. The needle centers when the aircraft is on course and deflects toward the side of the selected course the aircraft needs to fly to in order to return to it.
Plain English
A needle that tells you whether you are left or right of the course line you have chosen, and roughly how far off you are. Center the needle and you are on course.
Context Anchor
Seen on navigation instruments and glass-panel displays when flying a selected course, especially during instrument flying.
Derivation
From 'course' (the path you intend to fly), 'deviation' (from Latin deviare, 'to turn off the way'), and 'indicator' (something that shows or points out). Together: a device that points out how far you have turned off your intended path.
Why Pilots Care
The CDI is the primary visual cue for staying on a planned course. Misreading it, or chasing the needle the wrong way, leads directly to course excursions, missed approaches, or busted airways.
Intuition Check
Do not think of the CDI as the whole navigation system. It is the part of the display that shows left-or-right course error.
Example Sentence 1
After intercepting the airway, the pilot adjusted heading until the CDI centered.
Example Sentence 2
The pilot adjusted heading to center the CDI before reaching the final approach fix.