The measure that decides how clean an image looks, and the reason all noise lives in the shadows. A brightly lit pixel might put out five volts; a shadowed one, a hundredth of a volt. The same tiny error that disappears against five volts is ten times the entire signal of the dark pixel. Bigger pixels gather more light and keep the signal far above the error, which is why low-light cameras favor fewer, larger pixels.
A glow or halo that spreads around a small bright subject photographed against darkness, such as a lit flag at night. It is a limit of the sensor, not of technique: very bright light overfills small pixels and spills into their neighbors. Larger sensors with larger pixels contain it better, and the demosaicing process helps limit the spread.
The sensor format matching a 35 mm film frame. For low light and the night sky it is the practical minimum: bigger pixels keep the signal above the noise. Notice that the finest low-light bodies carry only 20 to 25 megapixels rather than 60 or more. Fewer, larger pixels are the point, not a compromise.