u

u UK [juː] / US [ju] or U UK / US noun [countable/uncountable]
Word forms "u":
singular u plural u's
Word forms "U":
singular U plural U's

1) the 21st letter of the English alphabet. U is a vowel.

2) ungraded: a mark given to a student, showing that a test or piece of work that they have done is extremely bad

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See:

U I, U-boat, U-turn