Antecedent

In grammar, an antecedent is a word or phrase that is repeated by another word or phrase. An antecedent is most often used with relative pronouns, words like "what", "that", and "who". The pronoun usually begins the relative clause and the antecedent is in the main clause. 
Examples.
"I want a book that I can read".
In the example, the relative pronoun 'that' refers back to the antecedent, the word 'book'. , the main clause is "I want a book" and the relative clause is "that I can read". 
Some more examples: 
The relative pronouns are: , , , , .