Anything that comes before something else of a similar form.
...or, if used as an adjective:
In grammar, the "antecedent" is the noun, pronoun, or noun phrase that a pronoun replaces.
The definition of a "pronoun" is "a word that takes the place of a noun". A pronoun will also take the place of another pronoun or a noun phrase.
When George got to 19th Street, he got off the train.
- The noun "George" is the antecedent of the pronoun "he."
You and I can finish this if we work together.
- The pronouns "you and I" are the antecedents of the pronoun "we".
The vacation cottage was so nice that we may rent it again.
The noun phrase 'the vacation cottage" is the antecedent of the pronoun "it".