means The bending of light (or any wave) as it passes from one medium into another of different density, which changes its speed and direction.

from From Latin 'refractus,' the past participle of 'refringere' — to break up or break open — built from 're-' (back) and 'frangere' (to break), the same root behind 'fracture' and 'fragment.' The idea is that the wave's path is, in a sense, broken at the boundary, snapped onto a new line as it crosses into glass or water. The scientific sense settled in English in the 1600s, as students of optics worked out exactly how light bends.