A partially specified but unnamed person.
"Someone left their umbrella on my porch when I wasn't looking."
In plain English: Someone is a person who is not specifically named or identified.
"Someone knocked on the door while I was in the kitchen."
Usage: Use someone to refer to an unspecified individual when their identity or name is unknown, irrelevant, or intentionally omitted from the sentence. This term functions as both a subject and object in standard grammar without requiring any special article usage like "the" unless specifically pointing out that particular unnamed person previously mentioned.
Some person.
"If someone calls later, tell them I'm out of town."
The word "someone" comes from Middle English, where it was formed by combining the words "some" and "one." It originally meant a particular person among others, much like its modern usage.