when Definition

1. Used in questions, indirect questions and statements: at what timeG; during what period?; at which time.

1. At the time, or during the period, that.
2. As soon as.
3. At any time that; whenever.
4. But just then.
5. At which time.
6. In spite of the fact that; considering that

Example: Why just watch when you could be dancing?

1. What or which time.

Example: They stayed talking, until when I can't say
Example: since when she hasn't spoken to me
2. Used as a relative pronoun: at, during, etc which time.

Example: an era when life was harder

colloq
Used specifically when pouring a drink: tell me when to stop.