Use of the definite article in chapter headings?

In a grammar book we have the headings like...

THE SENTENCE
THE NOUN
THE ADJECTIVE

What is the meaning of using the definite article in these headings?

The definite article "the" is used in these headings to refer to specific, known entities within the category being discussed. For example, the heading "THE NOUN" refers to a specific category of words (nouns) that have already been introduced and defined in the grammar book. Using the definite article in these headings serves to emphasize the specific, established nature of the category being discussed.