D , d plural D's, d's
1 [uncountable and countable]SLA
the fourth letter of the English alphabet
2 [uncountable and countable]APM
the second note in the musical scale of C major or the musical key based on this note
a mark given to a student's work to show that it is not very good:
I got a D in history last semester.
the number 500 in the system of roman numerals