d or D 
/ˈdi:/ noun
plural d's or ds or D's or Ds /ˈdi:/ noun 1 : the fourth letter of the English alphabet [count] ▪ Many people who are learning to read confuse b's and d's. [noncount] ▪ names that start with d 2 : a musical note or key referred to by the letter D : the second tone of a C-major scale [count] ▪ play/sing a D [noncount] ▪ The song is in the key of D. 3 [count] : a grade that is given to a student for doing poor work
▪ She got a D on her chemistry test. 4 : the Roman numeral that means 500
▪ CD [=400]
▪ She got a D on her chemistry test.
▪ CD [=400]
