Meaning of “someone” - Learner’s Dictionary

someone

pronoun us uk /ˈsʌmwʌn/ also somebody
Extra Examples
I picked up someone else's book by mistake.That's a fairly normal weight for someone of your height.It was fortunate that someone was available to take over.I stopped someone in the street to ask for directions.There's only so much help you can give someone.

A2 used to refer to a person when you do not know who they are or when it is not important who they are:

There's someone at the door.
Will someone please answer the phone?
someone else

A2 a different person:

Sorry, I thought you were talking to someone else.

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