The word, phrase, or clause referred to by a pronoun. The AP language exam occasionally asks for the antecedent of a given pronoun in a long, complex sentence or in a group of sentences.
Examples:
In logic, an antecedent is the first half of a hypothetical proposition.
In grammar, an antecedent is the word to which a pronoun refers.
A musical phrase (music) may be an antecedent or consequent phrase.
source: http://en.wikipedia.org/wiki/Antecedent