ETYM Old Eng. when, whan, whenne, whanne, AS. hwaenne, hwanne, hwonne; akin to OS. hwan, OD. wan, Old High Germ. wanne, German wann when, wenn if, when, Goth. hwan when, and to Eng. who. Related to Who.
1. At what time; used interrogatively.
2. At what time; at, during, or after the time that; at or just after, the moment that.
3. Which time; then; -- used elliptically as a noun.