The word "which" is a pronoun used to introduce a clause that provides additional information about a specific noun. It helps clarify or specify one or more items from a group. For example, in the sentence "I chose the book, which is on the table," the word "which" refers to the specific book being discussed. In English grammar, "which" is often used in non-restrictive clauses, meaning the information it introduces is not essential to the main point. For instance, "The car, which is red, belongs to John." Here, the clause adds detail about the car but is not necessary to understand who owns it.