orbit
An orbit is the looping path one object traces around another, held in place by gravity. Earth circles the Sun, the Moon circles Earth, a satellite circles the planet — each one falling endlessly around its partner without ever crashing down.
The trick is a balance of two things: the body is racing sideways fast enough that, as gravity tugs it inward, it keeps missing the ground. Imagine throwing a ball so hard it curves with the Earth and never lands — that sideways speed is what an orbit lives on. Too slow and its path dips back down to the surface; too fast and it escapes into space.
And orbits are not perfect circles. Most are gentle ellipses — slightly squashed ovals — so a planet swings a little closer to the Sun at one point and a little farther at another, speeding up when near and slowing down when far.
The International Space Station circles Earth once every 90 minutes, racing at about 28,000 km/h — fast enough to keep falling around the planet instead of into it.
The word comes from the Latin orbita, "wheel track" or "course." In 1609 Johannes Kepler showed that planets move in ellipses, not the perfect circles astronomers had assumed for two thousand years — and Newton later explained why: the same gravity that drops an apple keeps the Moon on its leash.