when 
(wĕn, hwĕn)
(wĕn, hwĕn)adv.
At what time: When will we leave?
conj.
1. At the time that: in the spring, when the snow melts.
2. As soon as: I'll call you when I get there.
3. Whenever: When the wind blows, all the doors rattle.
4. During the time at which; while: When I was young, I was sick all the time.
5. Whereas; although: She stopped short when she ought to have continued.
6. Considering that; if: How can he get good grades when he won't study?
pron.
1. What or which time: Since when has this been going on?
2. At or during the time that: Where were you on the night when the murder took place?
n.
The time or date: Have they decided the where and when?
[Middle English, from Old English hwenne; see kwo- in the Appendix of Indo-European roots.]
Usage Note: In informal style when is often used after forms of be in definitions: A dilemma is when you don't know which way to turn. Although useful, this construction is widely regarded as incorrect or unsuitable for formal discourse. In formal style such definitions should be recast to eliminate is when, either by supplying a generic term that may be modified by a restrictive adjective clause (A dilemma is a situation in which you don't know which way to turn) or by making the when-clause adverbial (You are in a dilemma when you don't know which way to turn). When is acceptable, however, when a noun phrase that denotes a point in time is being defined or described: The best time to drink this tea is when you are sleepy.