Class of words used to express spatial or temporal relations or mark various semantic roles
Adpositions are a class of words used to express spatial or temporal relations (in, under, towards, behind, ago, etc.) or mark various semantic roles (of, for). The most common adpositions are prepositions (which precede their complement) and postpositions (which follow their complement). An adposition typically combines with a noun phrase, this being called its complement, or sometimes object.
https://en.wikipedia.org/wiki/Adposition