1.When you have a revelation on Youtube. This also usually includes watching the video over and over and quoting it when it is not appropriate.
2. Can also mean when there is a youtube video that everyone has seen except you so you finally get to watch it
1. Dude, I totally watched "Jesus is my friend" on youtube and I had a youtube-iphany.
2. Jeez I finally watched "my new haircut" and after my youtube-iphany I understand the jokes you guys were making.