gravitational wave
A gravitational wave is a ripple in the fabric of space and time itself, set off when very heavy objects whip around or smash together — like two black holes spiraling into each other. As the wave rolls past, it gently stretches space in one direction and squeezes it in another, then snaps back.
Einstein predicted these waves a century ago, but they're so faint that no one could catch one until 2015, when a giant instrument called LIGO felt a tremor from black holes that collided over a billion years ago. The result was announced to the world in 2016.
The effect is mind-bendingly tiny: as a wave passes through you, your height changes by far less than the width of a single atom. That's why detecting one took some of the most sensitive machines humans have ever built.
Don't confuse a gravitational wave with a 'gravity wave' — physicists use that second term for ordinary waves like ripples on a pond or air rolling over mountains. Same word, completely different thing.