1. A word that indicates or shows the relationship between its object (the noun or pronoun following it) and other words in a sentence: The most common prepositions show direction, as "through the door", time, as"during his sleep", and possession, like "with Susan's mother".

In the sentence, "Please put the glass on the table", the word "on" is a preposition that indicates the relationship of the "glass" with the "table".

In "It will be time to catch the train in one hour", "in" is the preposition.

2. A word that, when combined with pronouns, nouns, or noun phrases, indicates a position in time or spaces, motions, agencies, relationships, or purposes: Prepositions govern their objects by deciding which words or phrases their objects may be associated with. "The houses by the road were all sold yesterday." "Road" is the object of the preposition and "by" determines that "road" will be associated with "houses".

3. Etymology: from Latin praepositionem, praepositio, "a putting before", from praepositus, past participle of praeponere, "to put before"; from prae, "before" + ponere "to put, to place, to set".