A definite article is a part of speech which is used in front of a noun. Definite articles are used as a specifier when the noun is known either because it is unique, or because it has been previously introduced in context. They are also used in some specific settings such as in references to certain types of geographic locations, before superlatives, and so forth. In English, there is only one definite article, and it is “the.” Closely related is the indefinite article, “a” or “an” in English.
One case in which a definite article might be used is when a noun has been introduced in context, when one wants to refer to that specific noun again. For example, one might say “As I walked up to the house, I could see a gray cat asleep on the windowsill. The cat yawned and stretched when I knocked on the door.” In this case, “the cat” in the second sentence is assumed to be the same cat who was asleep in the first sentence, because no other cats have been mentioned. “The” is used as a specifier, meaning “a particular cat,” instead of a cat in general.
People can also use a definite article when a noun is well understood or considered to be unique, as in “the Prime Minister will be giving a speech later” or “the moon is big tonight.” In the first sentence, the speaker is referring not to a generic prime minister, but to a particular Prime Minister, and the speaker assumes that the listener knows which one is being discussed. In the case of the moon, since Earth has only one moon, the speaker uses the definite article to refer to it even when it has not been introduced in the context of the conversation, because the speaker could only be talking about one moon in particular.
Although the definite article primarily acts as a determiner, being used to refer to a particular example of a noun, it can also be used in some other contexts. For example, when superlatives are used, “the” is usually used with them, as in “the biggest town in the county is Freetown” or “the highest building in the city is...” Definite articles also crop up in geography, where they are used to refer to particular geographic features, such as oceans, mountain ranges, and deserts, like the Atlantic Ocean, the Alps, and the Sahara.
As a general rule, the indefinite article is used when one is introducing a noun, or referring to a generic noun, while the definite article is used when talking about a particular noun. For example, one might say “will you be renting a car while you visit the city?” as opposed to “the keys to the rental car are on the table.” In the first sentence, one is referring to a generic car, while in the second, one is talking about a specific car. Likewise, “I am thinking of planting a tree in the garden” might be a topic of conversation among a group of gardeners, and another gardener might add “I really like the flowering cherry I planted last spring,” referring to a specific tree.