What is the Definition of Antecedent?

In grammar, an antecedent is a noun or noun phrase that is replaced by a pronoun either in that same sentence or in a sentence shortly thereafter. For example look at these sentences. ‘The bike was green with a whte seat. It is a three speed and is good for mountain trails.’ The antecedent is ‘bike’ because we replaced that word with the word ‘it’ in the second sentence while we were still talking about the bike.