means The lengthening of light's wavelength toward the red end of the spectrum, observed when a source moves away from us, used most famously to show that distant galaxies are receding and the universe is expanding.

from A transparent compound of English 'red' and 'shift,' coined by astronomers in the early 20th century to describe exactly what they saw: spectral lines sliding toward the red, longer-wavelength end. The companion term 'blueshift' (for objects approaching) follows the same logic. 'Red' is among the oldest color words in English, traceable through Old English 'rēad' back to a Proto-Indo-European root for redness shared with Latin 'ruber' and many cousins; 'shift' comes from Old English 'sciftan,' to arrange or divide, which drifted over centuries toward the sense of moving or changing position.