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 reconstruction of full color from the Bayer mosaic. Each pixel behind its single filter is a pixel well that knows one color's brightness and nothing else. The processor walks the entire sensor and, for every pixel, reads it together with its eight neighbors, weighs the reds, greens, and blues among them, and assigns the center pixel its final color. On a 50-megapixel sensor that happens fifty million times, faster than you can think about it.
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.