Definition of "adposition"

Throughout this book, I have assumed that adpositions (prepositions and postpositions) are not lexical categories, but rather functional categories. […] While this view of adpositions is far from unprecedented, it runs contrary to the more standard generative treatment, championed by Jackendoff (1977: 31-33), in which adpositions constitute a fourth lexical category, filling out the logical space of possibilities defined by the two binary-valued features + / − N {\displaystyle +\!/\!\!-\!\!{\mathsf {N}}} and + / − V {\displaystyle +\!/\!\!-\!\!{\mathsf {V}}} .

2003, Mark C. Baker, Lexical Categories: Verbs, Nouns and Adjectives, Cambridge University Press, page 303

It is well known in West African linguistics that languages in this broad sense display adpositions that emerge out of these two sources, namely nouns and verbs.

2008, Amani Bohoussou, Stavros Skopeteas, Grammaticalization of spatial adpositions in Nànáfwê, Elisabeth Verhoeven, Stavros Skopeteas, Yong-Min Shin, Yoko Nishina, Johannes Helmbrecht (editors), Studies on Grammaticalization, Walter de Gruyter (Mouton), page 77

By establishing adpositions as a constantly referred to but never really demonstrated language category, this book has provided a basis for the theory of the linguistic category. […] Adpositions could be considered a clear-cut category if one relied on syntax only, for one simple reason: the are specialized in function-marking.

2010, Claude Hagège, Adpositions, Oxford University Press, page 332