The word "which" is a pronoun used to introduce a clause or phrase that provides additional information about a specific noun. It helps to clarify or specify one or more items from a group. For example, in the sentence "I have two books, which are both interesting," the word "which" refers to the two books mentioned. In English grammar, "which" is often used in questions and relative clauses. In questions, it helps to ask for specific information, such as "Which color do you prefer?" In relative clauses, it connects additional details, as in "The car, which is red, belongs to John."