Pronunciation: (hwen, wen; unstressed hwun, wun),[key] —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. 1. at what time: to know when to be silent. 2. at the time or in the event that: when we were young; when the noise stops. 3. at any time; whenever: He is impatient when he is kept waiting. 4. upon or after which; and then: We had just fallen asleep when the bell rang. 5. while on the contrary; considering that; whereas: Why are you here when you should be in school?
—pron. 1. what time: Till when is the store open? 2. which time: They left on Monday, since when we have heard nothing.
—n. the time of anything: the when and the where of an act.