The is the definite article, since it points out a particular individual, or group, or class.
An or a is the indefinite article, because it refers to any one of a group or class of things.
An and a are different forms of the same word, the older ān.
USES OF THE DEFINITE ARTICLE.
Reference to a known object.
177. The most common use of the definite article is to refer to an object that the listener or reader is already acquainted with; as in the sentence,—
Don't you remember how, when the dragon was infesting the neighborhood of Babylon, the citizens used to walk dismally out of evenings, and look at the valleys round about strewed with the bones?—Thackeray.
NOTE.—This use is noticed when, on opening a story, a person is introduced by a, and afterwards referred to by the:—
By and by a giant came out of the dark north, and lay down on the ice near Audhumla.... The giant frowned when he saw the glitter of the golden hair.—Heroes Of Asgard.
With names of rivers.