Prepositional is a grammar word. It describes things that belong to a preposition — a small word we put before a noun, a pronoun, or an -ing form to show place, time, direction, and similar relations. In the trees in the park, in is the preposition, and in the park is a prepositional phrase. Other everyday chunks work the same way: on the table, at three o'clock, to school, from home, with a friend. Teachers use prepositional when they point at that whole little chunk, not at a verb or an adjective. You do not need this word to buy a ticket; you need it to talk about English itself. Though actually, students sometimes think prepositional is the name of the small word. The small word is the preposition. Prepositional only describes the words and phrases that go with it.

介系詞的（台灣課內用語；中國大陸多說「介詞的」）