Flash is a sudden, brief burst of light. Lightning creates a brilliant flash across the sky. When a photographer takes a picture in dim light, a camera's flash produces a quick, bright light to illuminate the subject.

The word also describes anything that happens very quickly, like a sudden thought or moment. When understanding comes in a flash, you grasp something instantly. A flash flood is a dangerous flood that happens suddenly after heavy rain, giving people little time to reach safety. If someone moves in a flash, they're moving with remarkable speed.

As a verb, to flash means to shine briefly or to show something quickly. Traffic lights flash to warn drivers. Someone might flash a badge to prove their identity, or flash a smile at a friend across the room.

You might also hear “flash in the pan,” meaning something that seems impressive at first but doesn't last, like a student who scores perfectly on one test but then does poorly on later tests.