The word "when" is an adverb that is used to inquire about the time something happened, will happen, or is happening. It can also introduce a clause that indicates a particular time. Essentially, it's a term that helps to establish time relationships between actions or events.
The word "when" comes from Old English "hwænne," which is derived from Proto-Germanic "*hwanai," meaning "at what time." It has been used in various forms throughout the history of the English language.