The definiteRelating to a specific or determined person, thing, idea or time. E.g.: I know the guy you are talking about. ("the guy you are talking about") articleAny of the words "a", "an" and "the", used to limit or give definiteness to a noun. "A" and "an" are indefinite articles, while "the" is a definite article. E.g.: A man was standing on the platform. ("a", "the") can be used with proper names if they would not have a clear reference without an additional phrase.

The Mr Brown in the blue shirt is my neighbour. The other one I don’t know.

The definite article with names of places and institutions
The definite article (THE) The definite article with titles and positions