When describes a point in time, and the circumstances under which something occurred.(adverb)
An example of when is the time at which you go to the store.
When is defined as information such as what time, what point or an alternative.(conjunction)
An example of when is the point in time that the runners started running.
See when in Webster's New World College Dictionary
Origin: ME whenne < OE hwænne, akin to Ger wann, when, wenn, if, akin to hwa, who: see what
See when in American Heritage Dictionary 4
Origin: Middle English
Origin: , from Old English hwenne; see kwo- in 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 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).
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