Yes, the word robber is a noun, singular, common, abstract noun; a word for a person (or creature) who robs.One definition for noun is:the word class that can serve as the subject or object of a verb, the object of a preposition, or in appositionWhich means that yes, robber Is a noun since it's the subject of a verb. However this is not a strict rule, as you can have a subjects or objects spanning multiple words. Such as a 'a fat robber' though this is a complete subject 9or object) not all of it is a noun.