Burn-in
Also called OLED burn-in · screen burn
Burn-in is a permanent faint image of something that stayed on an OLED screen for a long time, such as a taskbar or a game HUD. It happens because pixels that are lit more often wear faster than the ones around them.
It is easiest to see on a full screen mid gray or on solid red, green and blue. A shape that fades within minutes is image retention instead.
In practice
Burn-in affects OLED screens because each pixel ages as it is used. Static parts of the screen age faster than the rest. Taskbars, game HUDs and channel logos are the usual culprits. Modern OLED screens include protection such as pixel shift and compensation cycles, so burn-in is far less common than it once was.
How to check
Show full screen gray in the dead pixel test and look for faint shapes where static content usually sits. Then try red, green and blue, since one color often shows wear more clearly. If a shape fades after the screen runs its standby compensation cycle, it was image retention rather than burn-in.