"Someone" can be used to denote the presence of an unknown, or unnamed person:

> eg "*there is somebody at the door*".

It might be helpful to note that the opposite of this definition would be either "*nobody*" or "*no one*":

> "There is nobody at the door".

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The second definition of "someone", which is what your question is about, is "*a person of significance*", or "*someone noteworthy*".

> eg "*One day I'm going to be somebody*".

Interestingly, the words "no one" and "nobody" are also used to express that somebody is *not* significant:

> He's a nobody.