A grammatical word which occurs with a noun or phrase to express the relation it has to other parts of a sentence.

Prepositions are essential to sentences because they provide additional and necessary details, such as where something takes place (such as at the shop), when or why something takes place (such as before dinner), or general descriptive information (such as the man with the broken leg).
Prepositions go ‘before’ the thing they modify (usually a noun or a noun phrase) – in some languages these go after the thing, and are called postpositions.
In some languages prepositions are separate words, where as, in others they attach to the word they refer to.
In English prepositions can be presented as a prepositional phrase, such as on top of,  or in between.