Relativity
Etymology
From "relative", via Latin "relatus" (carried back, referred). Einstein’s 1905 special theory and 1915 general theory made the abstract word the name for how physical quantities shift with the observer’s frame.
noun · /rel-uh-TIV-ih-tee/
Einstein’s discovery that measurements of time, space, and mass depend on the observer’s motion, and that gravity is the curving of spacetime.
What it names
The collapse of absolute time: there is no universal “now,” and a fast-moving clock genuinely ticks slower — not an illusion but a fact of the universe.
Example
GPS satellites must correct for relativity, or their clocks would drift enough to throw your location off by miles each day.
Related idea
Relativity reshaped our picture of gravity — no longer a pull across empty space, but objects following the curves that mass presses into spacetime.