left-right indicator

course correction

i. Allowances made for deviations, variation, and drift.
ii. The alteration of a course to get back on track. course deviation indicator (CDI) An indicator used with VOR (very high frequency omnidirectional radio-range), LORAN (long-range navigation), GPS (global positioning system), or other area navigation systems to indicate the direction and relative amount of deviation from the desired course. Also called a left-right indicator.