Source : Webster's Revised Unabridged Dictionary (1913)

When \When\, adv. [OE. when, whan, whenne, whanne, AS.
   hw[ae]nne, hwanne, hwonne; akin to OS. hwan, OD. wan, OHG.
   wanne, G. wann when, wenn if, when, Goth. hwan when, and to
   E. who. ????. See {Who}.]
   1. At what time; -- used interrogatively.

            When shall these things be?           --Matt. xxiv.
                                                  3.

   Note: 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.                        --Shak.

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

               Come hither; mend my ruff: Here, when! thou art
               such a tedious lady!               --J. Webster.

   {When as}, {When that}, at the time that; when. [Obs.]

            When as sacred light began to dawn.   --Milton.

            When that mine eye is famished for a look. --Shak.