when

[hwen, wen; unstressed hwuhn, wuhn]
adverb
1.
at what time or period? how long ago? how soon?: When are they to arrive? When did the Roman Empire exist?
2.
under what circumstances? upon what occasion?: When is a letter of condolence in order? When did you ever see such a crowd?
conjunction
3.
at what time: to know when to be silent.
4.
at the time or in the event that: when we were young; when the noise stops.
5.
at any time; whenever: He is impatient when he is kept waiting.
6.
upon or after which; and then: We had just fallen asleep when the bell rang.
7.
while on the contrary; considering that; whereas: Why are you here when you should be in school?
pronoun
8.
what time: Till when is the store open?
9.
which time: They left on Monday, since when we have heard nothing.
noun
10.
the time of anything: the when and the where of an act.

Origin:
before 1000; Middle English when(ne), Old English hwenne; cognate with German wann when, wenn if, when (compare Gothic hwan when, how); akin to who, what

1. when, whence ; 2. wen, when.
Dictionary.com Unabridged

when's

[hwenz, wenz]
1.
contraction of when is: When's the show over?
2.
contraction of when does: When's the next train leave?
3.
contraction of when has: When's he ever been an authority?

See contraction.
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
when (wɛn)
 
adv
1.  a.  at what time? over what period?: when is he due?
 b.  (used in indirect questions): ask him when he's due
2.  say when to state when an action is to be stopped or begun, as when someone is pouring a drink
3.  (subordinating) at a time at which; at the time at which; just as; after: I found it easily when I started to look seriously
4.  although: he drives when he might walk
5.  considering the fact that: how did you pass the exam when you'd not worked for it?
6.  at which (time); over which (period): an age when men were men
 
n
7.  (usually plural) a question as to the time of some occurrence
 
usage  When should not be used loosely as a substitute for in which after a noun which does not refer to a period of time: paralysis is a condition in which (not when) parts of the body cannot be moved

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Etymonline
Word Origin & History

when
O.E. hwænne, hwenne, hwonne, from Gmc. pronomial stem *khwa- (cf. O.S. hwan, O.Fris. hwenne, M.Du. wan, O.H.G. hwanne, Ger. wann "when"), from PIE interrogative base *qwo- (see who).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Idioms & Phrases

when

In addition to the idioms beginning with when, also see cross a bridge when one comes to it; push comes to shove, when.

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
Correct tire inflation can only be obtained when weight of car is known.
When feelings of kinship transcend the species boundary.
When it comes to courtship, mankind has nothing on the animal kingdom.
Bonobos have an impressive vocabulary, especially when it comes to snacks.
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