when

UK: /ˈ^hwɛn/

US: /hwɛn, wɛn; unstressed hwən, wən/

US: (hwen, wen; unstressed hwən, wən)

WordReference Random House Learner's Dictionary of American English © 2025

when /hwɛn, wɛn; unstressed hwən, wən/   adv. 

1. (used to introduce a question) at what time or period?:
   When will they arrive?
2. (used to introduce a question) under what circumstances?:
   When is an apology in order?

conj. 

3. at what time:
   He knows when to be silent.
4. at the time that:
   when we were young.
5. whenever:
   The dog barks when the doorbell rings.
6. as soon as:
   Stop the car when the light turns red.
7. whereas;
   while on the contrary:
   Why are you here when you should be in school?

pron. 

8. what or which time:
   Since when have you been teaching?

n. [countable* usually singular]

9. the time of something:
   the where and when of the accident.

WordReference Random House Unabridged Dictionary of American English © 2025

when  (hwen, wen; unstressed hwən, wən), adv. 

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?

conj. 

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?

pron. 

8. what time:
   Till when is the store open?
9. which time:
   They left on Monday, since when we have heard nothing.

n. 

10. the time of anything:
    the when and the where of an act.

• bef. 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

Collins Concise English Dictionary © HarperCollins Publishers::

when / wɛn/ adverb

1. at what time? over what period?: when is he due?
2. (used in indirect questions): ask him when he's due
3. say when ⇒ to state when an action is to be stopped or begun, as when someone is pouring a drink
4. (subordinating) at a time at which; at the time at which; just as; after: I found it easily when I started to look seriously
5. although: he drives when he might walk
6. considering the fact that: how did you pass the exam when you'd not worked for it?
7. at which (time); over which (period): an age when men were men

noun

1. (usually plural) a question as to the time of some occurrence

Etymology: Old English hwanne, hwænne; related to Old High German hwanne, hwenne, Latin cumUSAGE
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