The word "which" is a pronoun used to ask questions or to introduce relative clauses. It typically refers to items from a set or choices. For example, when asking someone to specify a single option among many, you might say, "Which one do you prefer?" It helps in clarifying or giving additional information about something previously mentioned.
The word "which" originates from Old English "hwilc," which meant "what kind of" or "which one." The root can also be traced to Proto-Germanic "*hwi-likaz," which has similar meanings in related languages.