We found 16 dictionaries that define the word a one:

General (13 matching dictionaries)

1. A-one: American Heritage Dictionary of the English Language
2. A-one, A-one: Collins English Dictionary
3. A-one: Vocabulary.com
4. A-One, A-one, a-one: Wordnik
5. A one (Aone or A-1 or A number A+one+++(Aone+++or+++A-1+++or+++A+++number+++1)): The Wordsmyth English Dictionary-Thesaurus
6. A-ONE, A-One (TV channel), A-One (band), A One: Wikipedia, the Free Encyclopedia
7. A-one: Rhymezone
8. a-one: FreeDictionary.org
9. a-one: Mnemonic Dictionary
10. A-one: TheFreeDictionary.com
11. A one: Infoplease Dictionary
12. A-one, a one, A One: Dictionary.com

Art (1 matching dictionary)

13. A-one: Artist Search

Computing (1 matching dictionary)

14. A-one: Encyclopedia

Slang (1 matching dictionary)

15. a one: Urban Dictionary

Definitions from Wikipedia (A One) ▸ noun:  the sixteenth full-length studio album recorded by Japanese singer Ayumi Hamasaki. Phrases: pull a fast one, pull a fast one on, pulled a fast one, pulling a fast one, pulls a fast one, more... ▸ Words similar to a one ▸ Usage examples for a one ▸ Idioms related to a one ▸ Wikipedia articles (New!) ▸ Words that often appear near a one ▸ Rhymes of a one ▸ Invented words related to a one