The definite article in English is “the.”
A definite article signals that a noun refers to a particular, identifiable person, thing, or group. In “Please close the door,” the speaker assumes the listener can identify which door is meant. The article can refer back to something already mentioned, as in “I saw a film. The film was hilarious,” or to something unique in the relevant context, such as “the sun.”
English has two basic articles: “the” is definite, while “a” and “an” are indefinite. The choice between “a” and “an” depends on sound rather than spelling: “a university” begins with a consonant sound, but “an hour” begins with a vowel sound. “This” is a demonstrative determiner, not an article.
English normally uses the same form, “the,” with singular, plural, and uncountable nouns. It can therefore appear in “the book,” “the books,” and “the water.” However, English sometimes omits an article altogether, especially with plural or uncountable nouns used generally, as in “Books are useful” or “Water is essential.”