means The principle that measurements of space, time, and motion depend on the observer's frame of reference — most famously formalized in Einstein's theories where even time and mass bend with speed and gravity.

from From Latin 'relativus,' meaning 'having reference or relation,' built from 'relatus,' the past participle of 'referre' — 'to carry back, refer.' The plain word 'relative' had drifted through English for centuries meaning simply 'in relation to something else,' but 'relativity' got its electric charge in the early 20th century, when Einstein's theories made it shorthand for the unsettling idea that no observer holds the one true ruler or clock.