Sensor Cleaning
Removing dust from the sensor's cover glass, either by the camera's own vibrating filter or manually with a blower or swab.
In depth
Ultrasonic cleaning shakes loose particles free at every power cycle and handles most ordinary dust. Stubborn or oily marks need a manual clean: a blower first, then a correctly sized wet swab drawn once in each direction. The mirrorless design exposes the sensor at every lens change, which makes routine checking more important than it was on DSLRs.
Examples in practice
Photographing a plain white wall at f/22 and inspecting the file is the standard test for whether a clean is needed.