twenty-first

Short definition from Wiktionary

Adjective

1. 1
   The ordinal form of the number twenty-one.

Noun

1. 1
   The person or thing in the twenty-first position.
2. 2
   One of twenty-one equal parts of a whole, usually written ¹⁄₂₁.
3. 3
   A party to celebrate a twenty-first birthday.