'D-day' definitions:

Definition of 'D-day'

(from WordNet)
noun
Date of the Allied landing in France, World War II [syn: D-day, 6 June 1944]

Definition of 'D-day'

From: GCIDE
  • D-day \D-day\ n.
  • 1. (Mil.) the day designated for the beginning of a planned attack by a military force; in communications the day is often otherwise unspecified for security reasons; as, the day before D-day the troops will move into position. [PJC]
  • 2. 6 June 1944, the day of the landing in France, by troops allied against the Germans in World War II.
  • Syn: 6 June 1944. [WordNet 1.5]
  • 3. the day for commencement or execution of any elaborate planned activity; -- often used jocosely. [informal] [PJC]