What is the definition of the definite article THE?

The Word "the" specifies a certain unit or member of a group of objects or things. This compares with the non-specific or indefinite article, which is "a" or "an". In written text, the second reference to a non-specific object should directly refer to that as a specific one.

Examples : "The boy picked up a stick. He threw the stick at the fence." (that same stick)