Eureka moment

A Eureka moment is when you understand something important and you realize it. It can be an emotional experience. It can also be a world and or life changing idea.

See also:

http://science.discovery.com/brink/top-ten/eureka-moments/eureka.html

http://en.wikipedia.org/wiki/Eureka_(word)

http://en.wikipedia.org/wiki/Eureka_effect