Dark Emitting diode, a burned-out LED. Written up by Texas Instruments as an April Fool's joke.

DED is a three-letter abbreviation with multiple meanings, including:

Doctor of Education, correctly spelled D.Ed. or Ed.D.

District Electoral Division, a low-level territorial division in Ireland.

Dravidian Etymological Dictionary by T.Burrow and M. B. Emeneau

Dragging Equipment Detector, a piece of railroad equipment placed on the track used to detect any part of the train that is dragging on the ground — a type of Defect detector.

Deutscher Entwicklungsdienst, the German Development Service.