according to wikipedia, its burn in.
Plasma & LCD & OLED
Plasma displays are highly susceptible to burn-in, while
LCD-type displays are generally less so. Because of the more rapid luminance degradation of current organic compounds used in
OLED-type displays, OLED is even more susceptible to burn-in than plasma. In addition, the wide variation in luminance degradation with OLED
[1] will cause noticeable color drift over time (where one of the red-green-blue colors becomes more prominent).
Unlike displays with a common light source, the brightness of each OLED pixel fades depending on the content displayed. The varied lifespan of the organic dyes can cause a discrepancy between red, green, and blue intensity. This leads to image persistence, also known as
burn-in.