Meaning of “one” - Learner’s Dictionary

one

determiner uk us /wʌn/
PARTICULAR PERSON/THING

A2 used to refer to a particular person or thing in a group:

One drawback is the cost of housing in the area.
One of our daughters has just got married.
FUTURE TIME

B2 used to refer to a time in the future that is not yet decided:

We must have a drink together one evening.
TIME IN PAST

B2 at a particular time in the past:

I first met him one day in the park.
ONLY

B2 only:

He's the one person you can rely on in this place.
WITH ADJECTIVE mainly US

used to emphasize an adjective:

That's one big ice cream you've got there!
one or two

B1 a few:

I'd like to make one or two suggestions.
→ See also put sth to one side , be one step ahead (of sb)

(Definition of “one determiner” from the Cambridge Learner’s Dictionary © Cambridge University Press)