According to Dictionary.com, the use of "antecedent" as an adjective simply indicates something that is preceding or prior. One can describe an event as antecedent, in this sense. As a noun, the word indicates a prior situation, event or object. The noun form, "antecedents," means "ancestors."
The word also has meanings tied to more specialized areas of focus, such as grammar, logic and mathematics. Dictionary.com states that an antecedent in mathematics is the first term of a ratio or the first of two vectors in a dyad. An antecedent in grammar is any clause, phrase or word that a pronoun or other substitute replaces later in the same or in another sentence. In the sentence "Harry cooked a steak and he ate it," "Harry" is the antecedent of "he," and "steak" is the antecedent of "it." In logic, an antecedent is the conditional element in any proposition.