We found 17 dictionaries that define the word 13:

General (9 matching dictionaries)

1. 13: Vocabulary.com
2. 13: Wordnik
3. 13: Dictionary.com
4. 13: Cambridge Essential American English Dictionary
5. 13: Rhymezone
6. 13: Stammtisch Beau Fleuve Acronyms
7. 13: FreeDictionary.org
8. 13: Mnemonic Dictionary
9. 13: Merriam-Webster

Business (1 matching dictionary)

10. Bernstein's Dictionary of Bankruptcy Terminology (No longer online)

Computing (1 matching dictionary)

11. 13, 13 (number): Encyclopedia

Miscellaneous (1 matching dictionary)

12. 13: Acronym Finder

Religion (1 matching dictionary)

13. Postmodern Bible Dictionary (No longer online)

Science (1 matching dictionary)

14. 13: Eric Weisstein's World of Mathematics

Slang (1 matching dictionary)

15. 13: Green’s Dictionary of Slang

Sports (1 matching dictionary)

16. 13: Card Games

Tech (1 matching dictionary)

17. Basics of Space Flight Glossary (No longer online)

(Note: See thirteen as well.)

Definitions from WordNet (13) ▸ noun:  the cardinal number that is the sum of twelve and one ▸ adjective:  being one more than twelve Opposite: unlucky, fortunate, auspicious, favorable, beneficial Types: unlucky, bad luck, unfortunate, more... Phrases: pg 13, atomic number 13, carbon 13, trisomy 13, trisomy 13 syndrome, more... ▸ Words similar to 13 ▸ Usage examples for 13 ▸ Idioms related to 13 ▸ Wikipedia articles (New!) ▸ Popular adjectives describing 13 ▸ Popular nouns described by 13 ▸ Words that often appear near 13 ▸ Rhymes of 13 ▸ Invented words related to 13