We found 14 dictionaries that define the word 21:

General (9 matching dictionaries)

1. 21: Vocabulary.com
2. 21: Wordnik
3. 21: Dictionary.com
4. 21: Rhymezone
5. 21: FreeDictionary.org
6. 21: Mnemonic Dictionary
7. 21: Merriam-Webster

Computing (1 matching dictionary)

8. 21, 21 (number): Encyclopedia

Miscellaneous (1 matching dictionary)

9. 21: Acronym Finder

Slang (2 matching dictionaries)

10. American-Australian Slang Dictionary (No longer online)
11. The 21: Urban Dictionary

Sports (1 matching dictionary)

12. 21: Card Games

Definitions from WordNet (21) ▸ noun:  the cardinal number that is the sum of twenty and one ▸ adjective:  being one more than twenty Opposite: old, past, backward, retrograde, regressive Types: blackjack, poker, craps, roulette, slot machines, cards, more... Phrases: trisomy 21, atomic number 21, June 21, trisomy 21 syndrome, mpeg 21, more... ▸ Words similar to 21 ▸ Usage examples for 21 ▸ Idioms related to 21 ▸ Wikipedia articles (New!) ▸ Popular adjectives describing 21 ▸ Popular nouns described by 21 ▸ Words that often appear near 21 ▸ Rhymes of 21 ▸ Invented words related to 21