Adpositions are traditionally defined as words that “link to other words, phrases, and clauses” and that “express spatial or temporal relations.”
Adposition is a grammatical form.
The seven primary functions of adpositions are adpositional phrase head, noun phrase modifier, noun phrase complement, adjective phrase complement, verb phrase complement, adjunct adverbial, and disjunct adverbial. The six nominal functions of adpositions are subject, subject complement, direct object, object complement, indirect object, prepositional complement
Adpositions belong to a closed class of function words that lack any variation in internal structure.
Because prepositions vastly outnumber postpositions in the English language, most grammars group postpositions with prepositions and discuss both types of adpositions as prepositions.
Brinton, Laurel J. & Donna M. Brinton. 2010. The linguistic structure of Modern English, 2nd edn. Amsterdam: John Benjamins Publishing Company.
Hopper, Paul J. 1999. A short course in grammar. New York: W. W. Norton & Company.
Huddleston, Rodney. 1984. Introduction to the grammar of English. Cambridge: Cambridge University Press.