Flash on Definition 1 of 3Pro Tip 1/3Sudden Activation Refers to lights or devices turning on unexpectedly or quickly. We were startled when the room lights flashed on at midnight. Pro Tip 2/3Automatic Response Often used for systems that activate without manual input, like sensors. As we entered the building, the security lights flashed on. Pro Tip 3/3Visual Alert Signals attention to something important, like a warning or notification. The smoke detector's red light flashed on, signaling danger.