1) Closed-class word 2) Function word 
• (a.) Of or pertaining to a preposition; of the nature of a preposition.
Prep`o·si'tion·al adjective [ Confer French prépositionnel .] Of or pertaining to a preposition; of the nature of a preposition. Early. -- 
(Preposition (prepositional)) A small word or phrase that begins a longer adverbial phrase (called the object of the preposition) that acts to tell about place, time or manner and relate this aspect to some other word in the sentence, e.g. in, on, by, ahead of, near.…
[adj] - (linguistics) of or relating to or formed with a preposition
adjective of or relating to or formed with a preposition; `prepositional phrase`