Definition
An antecedent in English syntax is generally described as the noun an anaphor refers to. (Carnie, 2006, p. 27)
However, antecedents may also be expressions that give meaning to another pronoun, word, phrase, sentence or clause while its own meaning is determined in the surrounding linguistic context.
Examples

References
Carnie, A. (2006). Syntax: A generative introduction. John Wiley & Sons.