None at all.

Alternative spelling of twenty-one

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

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

(dated) Alternative spelling of oneself

(US) One thousand plus one.

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.

(In both the 12-hour clock and the 24-hour clock) The start of the second hour of the day; 1:00 a.m. (01:00). / (Only in the 12-hour clock) The start of the fourteenth hour of the day; 1:00 p.m. (13:00).