Meaning of “one” - Learner’s Dictionary

one

determiner us uk /wʌn/
Shopping is one of her favourite pastimes.A mobile phone is one of the perks of the job.She's one of our best students.Iron is one of the elements of the Earth's crust.He's one of the country's foremost experts on military law.
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)