d

D is the fourth letter in the English alphabet. (letter)

An example of d is the first letter of the word down.

See d in Webster's New World College Dictionary

or D

noun pl. d's, D's

the fourth letter of the English alphabet: from the Greek delta, a borrowing from the Phoenician
any of the speech sounds that this letter represents, as, in English, the (d) of dog
a type or impression for d or D
the fourth in a sequence or group
an object shaped like

adjective

noun

a Roman numeral for 500; with a superior bar (), 500,000 or less often, 5,000
☆ Educ. a grade indicating below-average work, or merely passing
Music
1. the second tone or note in the ascending scale of C major
2. a key, string, etc. producing this tone
3. the scale having this tone as the keynote

adjective

below average in quality

See d in American Heritage Dictionary 4

or D (dē)

noun pl. d's d's or D's also ds or Ds

The fourth letter of the modern English alphabet.
Any of the speech sounds represented by the letter d.
The fourth in a series.
Something shaped like the letter D.
D The lowest passing grade given to a student in a school or college.
Music
a. The second tone in the scale of C major or the fourth tone in the relative minor scale.
b. A key or scale in which D is the tonic.
c. A written or printed note representing this tone.
d. A string, key, or pipe tuned to the pitch of this tone.