The cardinal number immediately following thirty and before thirty-two; thirty plus one.

The cardinal number occurring after twenty and before twenty-two, represented in Roman numerals as XXI and in Arabic numerals as 21.

(card games) Blackjack. / (sports) A basketball-based game in which players attempt to score exactly twenty-one points.

(US) One thousand plus one.

The cardinal number following eighty and preceding eighty-two.

None at all.

Having only one arm; especially having lost an arm.

Having only one leg. / Of an audio signal: having a thin sound as a result of connection through only one wire of a circuit pair.

A plant, especially a tree.

(computing) The number obtained by subtracting a given n-digit binary number from 2ⁿ-1 (which yields the same result as the logical complement).