Exposure Simulation
A live-view or electronic-viewfinder mode that displays the scene at the brightness the current settings will actually record.
In depth
It is the single greatest practical advantage of mirrorless cameras: the exposure decision is made by looking rather than by predicting. The mode has to be switched off for studio flash, where the modelling light alone would render the finder almost black even though the flash exposure will be correct.
Examples in practice
Dialling in −2 EV for a silhouette, the viewfinder darkens immediately so the composition can be judged as the final image.