Which is the question word used to ask about a specific thing or person out of a group of options. It is commonly used to gather more information or to make a choice between different possibilities. For example, "Which book do you want to read?" or "Which color do you prefer?" In some cases, "which" can also be used as a relative pronoun to introduce a clause that provides more information about a previously mentioned noun. For instance, "I cant decide which dress to wear." Overall, "which" is a versatile word that helps to clarify and specify in English.