When

1. adv. Which time; then; — used elliptically as a noun.
2. adv. At what time; — used interrogatively.
3. adv. At what time; at, during, or after the time that; at or just after, the moment that; — used relatively.
4. adv. 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.

The word meanings were obtained from OPTED(The Online Plain Text English Dictionary), which is based on “The Project Gutenberg Etext of Webster’s Unabridged Dictionary” which is in turn based on the 1913 US Webster’s Unabridged Dictionary (See Project Gutenburg), as a text file.