Colloquialism for sudden inspiration or insight. A light-bulb moment is when that idea you've been wrestling with forever suddenly make sense. Sample uses:


"I really hated algebra, but one night I had a light-bulb moment and math was just easy after that."
"*blush* Oh. Sorry. The light bulb just went on. You're suggesting I leave you alone, aren't you? Yeah. I thought so." *shuffles away, embarrassed*


This phrase evolved first as a visual gag in cartoons. When a cartoon character has a bright idea, a light bulb will invariably appear from nowhere over their head and light up. The implication is that the clever idea is so bright it lights up the area.
Also, can be described as: an idea is so bright it illuminates the darkness of your mind - just like a light bulb!

From there, it became popular slang; people began to use it everyday speech, and it has become part of the culture.