WHEN

noun

1. The time at which something happens.

adverb

1. (interrogative) At what time? At which time? Upon which occasion or circumstance? Used to introduce direct or indirect questions about time.
2. At an earlier time and under different, usually less favorable, circumstances.
3. (relative) At which, on which, during which: often omitted or replaced with that.

pronoun

1. (interrogative) What time; which time.
2. The time that.