The short definition
A video is a sequence of still images called frames. At 30 frames per second, each frame stays on screen for about 0.033 seconds; at 60 frames per second, about 0.017 seconds. A flash frame is one of those frames (sometimes two) that looks completely different from the ones around it.
Editors also use the term for a very short burst of white, or a different shot, placed between two scenes. In everyday use it simply means an image that flashes by in a split second.
Why you almost never notice it
Your eyes and brain are built to follow motion, not to catch a single frame in a stream of dozens per second. When an image disappears as quickly as it appeared, you may sense that something changed without being able to say what.
Pausing at the right moment does not help much: the pause button rarely lands on the exact frame, and phone apps do not let you step through a video one frame at a time.
Where flash frames come from
Intentional effects: glitch and transition effects, impact flashes, jokes, easter eggs, or a brief logo or caption added by the creator.
Editing leftovers: a frame from another clip that stayed in by accident after a cut.
Compression: when a platform re-encodes a video, a very short frame can be blurred or merged with its neighbors, so the same flash frame can look different from one platform to another.
How to find one
By hand: pause the video and step forward one frame at a time. It works on a computer, but it is slow, and almost impossible on a phone.
Automatically: a tool can compare every frame with the frames before and after it. A flash frame stands out because it differs from both neighbors, while the video looks normal again right after. That is what FrameSonar does: paste the link of a public video and it shows you the frames that break from the rest.
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