when

Etymology

From Middle English when(ne), whanne, from Old English hwonne, from Proto-West Germanic *hwannē, from Proto-West Germanic *hwan, from Proto-Germanic *hwan (“at what time, when”), from Proto-Indo-European *kʷis (interrogative base). Cognate with Dutch wanneer (“when”) and wen (“when, if”), Low German wannehr (“when”), wann (“when”) and wenn (“if, when”), German wann (“when”) and wenn (“when, if”), Gothic 𐍈𐌰𐌽 (ƕan, “when, how”), Latin quandō (“when”). More at who. Interjection sense: a playful misunderstanding of "say when" (i.e. say something / speak up when you want me to stop) as "say [the word] when".

adv

1. (interrogative) At what time? At which time? Upon which occasion or circumstance? Used to introduce direct or indirect questions about time.
   When will they arrive?
   Do you know when they arrived?
   I don't know when they arrived.
   When they arrived is unknown.
   What words are used as interrogative pronouns? — Give examples. When are the words, what, which, and that, called adj. pron.? 1834, Samuel Kirkham, English Grammar in Familiar Lectures, page 117
2. At an earlier time and under different, usually less favorable, circumstances.
   He's mister high and mighty now, but I remember him when.
3. (relative) At which, on which, during which: often omitted or replaced with that.
   That was the day when the Twin Towers fell.
4. (fused relative) The time at, on or during which.
   I recall when they were called the Greys.
   Next year is when we elect a new mayor.
5. (informal, in definitions or explanations) A circumstance or situation in which.
   Love is when you can't get enough of someone.

conj

1. At (or as soon as) that time that; at the (or any and every) time that; if.
   Pavlov's dogs salivate when [i.e. at any and every time that] they hear a bell.
   When [i.e. at any and every time that] he speaks to her, he is always polite.
   Put your pencil down when [i.e. as soon as, at the moment that] the timer goes off.
   A player wins when [as soon as, or at any time that, if] she has four cards of the same suit.
   A student is disqualified when [as soon as, if] they cheat.
   Energy has seldom been found where we need it when we want it. 2013 July-August, Henry Petroski, “Geothermal Energy”, in American Scientist, volume 101, number 4
2. During the time that; at the time of the action of the following clause or participle phrase.
   They dream when [i.e. during the time that] they sleep.
   I'm happiest when [during the time that, or at any time that] I’m working.
   It was raining when I came yesterday.
   The game is over when the referee says it is.
   Be careful when crossing the street.
   When (you are) angry, count to ten before speaking or acting.
   The Baggies had offered little threat until the 28th minute, but when their first chance came it was a clear one. April 22, 2012, Sam Sheringham, “Liverpool 0-1 West Brom”, in BBC Sport
3. At what time; at which time.
   I am here till Friday, when [i.e. at which time] I leave for Senegal.
   I was just walking down the street, when [i.e. at which time] all of a sudden it started to rain.
   I am at London only to provide for Monday, when I shall use that favour which my Lady Bedford hath afforded me, of giving her name to my daughter; which I mention to you, […] 1839, John Donne, The Works of John Donne: Sermons, Letters, Poems, page 310
   He sat at the door of his kitchen watching, and seeing there was nothing else for it we buckled to and soon had the job done; when we were admitted to the kitchen and given a really good meal. 1929, Donald John Munro, The Roaring Forties and After, page 38
4. Since; given the fact that; considering that.
   I don't see the point of putting up Christmas decorations when I am the only person who is going to see them.
5. Whereas; although; at the same time as; in spite of the fact that.
   You're picking at your scabs when you should be letting them heal.
   He keeps changing things when the existing system works perfectly well.

pron

1. (interrogative) What time; which time.
   Since when do I need your permission?
   Homer, to whom the Muses did carouse A great deep cup with heav'nly nectar fill'd, The greatest, deepest cup in Jove's great house, (For Jove himself had so expressly will'd) He drank off all, nor let one drop be spill'd; Since when, his brain that had before been dry, Became the well-spring of all poetry. 1831 (published), John Davies, Orchestra Or, a Poem of Dancing, in Robert Southey, Select Works of the British Poets: From Chaucer to Jonson, with Biographical Sketches, page 706
   [This] we imagined might have been owing to some accidental condition of the system, or perhaps idiosyncracy; this led us to a second trial, but we experienced the same inconveniences, since when, we have altogether abandoned their use. 2012, Emile Letournel, Robert Judet, Fractures of the Acetabulum, Springer Science & Business Media: , →ISBN, page 385: So we combined the Kocher-Langenbeck and iliofemoral approach until 1965, since when we have combined the ilioinguinal and Kocher-Langenbeck approaches. 1833, William Potts Dewees, A Treatise on the Diseases of Females, page 495

noun

1. The time at which something happens.
   A good article will cover the who, the what, the when, the where, the why and the how.
   For the moment, suffice it to say that the stories told through the whens and hows of building a scene differentiate individual desires and needs more clearly than shared speech was up to then able to communicate. 2008, Paolo Aite, Lanscapes of the Psyche, Ipoc Press, page 151

intj

1. (often humorous) That's enough: a command asking someone to stop adding something, especially an ingredient or portion of food or drink; used in, or as if in, literal response to 'Say when'.
   When we go out to a restuarant, we're the guys who never say "when" when the waiter is grinding fresh pepper on our salads. 2004, Andy Husbands, Joe Yonan, The Fearless Chef: Innovative Recipes from the Edge of American Cuisine, page 83
   He keeps the bottle in the top bureau drawer; he takes it out, and two glasses, and pours. Say when. When, please. 2009, Margaret Atwood, The Blind Assassin, page 111
   Producers have the power to say "when" when the actress involved is too stressed to continue. That's responsible filmmaking. 2011, Fritz Allhoff, Dave Monroe, Porn - Philosophy for Everyone: How to Think With Kink
2. (obsolete) Expressing impatience. (Compare what.)