Webster's Revised Unabridged Dictionary · 1913

When

When , adverb

[Old English when, whan, whenne, whanne, Anglo-Saxon hwanne, hwanne, hwonne; akin to Old Saxon hwan, OD. wan, Old High German wanne, German wann when, wenn if, when, Gothic hwan when, and to English who. {not transcribed}. See Who.]

1.
At what time; -- used interrogatively.
When shall these things be? — Matt. xxiv. 3

See the Note under What, pron., 1.

2.
At what time; at, during, or after the time that; at or just after, the moment that; -- used relatively.
Kings may Take their advantage when and how they list. — Daniel
Book lore ne'er served, when trial came, Nor gifts, when faith was dead. — J. H. Newman
3.
While; whereas; although; -- used in the manner of a conjunction to introduce a dependent adverbial sentence or clause, having a causal, conditional, or adversative relation to the principal proposition; as, he chose to turn highwayman when he might have continued an honest man; he removed the tree when it was the best in the grounds.
4.
Which time; then; -- used elliptically as a noun.
I was adopted heir by his consent; Since when, his oath is broke. — Shakespeare
Come hither; mend my ruff: Here, when! thou art such a tedious lady! — J. Webster
When as sacred light began to dawn. — Milton
When that mine eye is famished for a look. — Shakespeare

When was formerly used as an exclamation of surprise or impatience, like what!

Collocations (1)
When as or When that , at the time that; when. [Obsolete]