Image burn-in on displays
When displaying the same image on a monitor (mostly on old CRT displays) the image remained vaguely visible even after the image was changed or even the display was turned off. This is called burn-in.
The term also got used for high power (which also means high heat, hence burn) testing period when using new components. Both for checking stability and in case of badly applied thermal grease to allow it to settle. Please do not use this tag for that. (note [stress-test] tag exists)