What part of speech is the word which?

The word "which" can be a few different parts of speech depending on its function in a sentence:

* Relative Pronoun: This is the most common usage. It introduces a relative clause that provides more information about a noun. For example: "The book which I read yesterday was very interesting."

* Interrogative Pronoun: When used in a question, it acts as an interrogative pronoun. For example: "Which book did you read yesterday?"

* Adjective: It can sometimes act as an adjective modifying a noun. For example: "Which way should we go?"

So, it's best to determine the part of speech based on its specific context in the sentence.