Definitions

1. noun

a single person or thing

“She’s the only one who knows the secret.”

2. adjective

being one in number, amount, or quantity

“I have one book on the shelf.”

3. adverb

used to indicate a single time or occasion

“I’ll meet you at one o’clock.”

4. numeral

representing a single quantity or amount

“The recipe calls for one cup of flour.”

5. pronoun

used as a substitute for a noun, especially in referring to a single person or thing

“Who’s one?”