A member of a closed word class, including of and in, which grammatically link other elements together, and generally expresses relations in space, in time, or in the mind.
A preposition links a following noun, pronoun or noun phrase to some other word in the sentence. Prepositions often describe locations or directions, but can describe other things, such as relations of time.
Words like before or since can act either as prepositions or as conjunctions.
- Tom waved goodbye to Christy.
- She’ll be back from Australia in two weeks.
- I haven’t seen my dog since this morning.
Contrast: I’m going, since no-one wants me here! [conjunction: links two clauses]