Gravitational Wave
Definition
Ripples in spacetime itself from colliding black holes — first directly heard by LIGO in 2015.
Technical definition
Propagating perturbations of the spacetime metric traveling at c, predicted by general relativity. Detected mergers show strain h = ΔL/L ~ 10⁻²¹.
Example
GW150914: two black holes 1.3 billion light-years away merged, converting 3 solar masses into pure spacetime ripple — briefly outshining every star.
Common confusion
Gravitational waves aren't waves IN a medium like ocean waves — they are traveling distortions of spacetime geometry itself.