The shifting of emitted electromagnetic radiation (such as visible light) towards the less energetic red end of the electromagnetic spectrum when a light source is moving away from the observer. This occurs as the wavelengths of lightstretch as an object moves away (as opposed to being squashed by an approaching object), similar to the familiar Doppler effect on sound waves. Among other things, it can be used as a measure of the speed with which galaxies throughout the universe are moving away from us.