Definition of when :
1. At what time; - used interrogatively.
2. At what time; at, during, or after the time that; at or just after, the moment that; - used relatively.
3. Which time; then; - used elliptically as a noun.
4. 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.