D, or d, is the fourth letter of the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide., 1. The fourth letter of the modern English alphabet. 2. Any of the speech sounds represented by the letter d. 3. The fourth in a series. 4. Something shaped like the letter D. 5. D The lowest passing grade given to a student in a school or college., D meaning: 1 : the fourth letter of the English alphabet; 2 : a musical note or key referred to by the letter D the second tone of a C-major scale, , , .