orbital period

The orbital period is simply the time it takes to go all the way around once: one lap of the Moon around the Earth, one lap of the Earth around the Sun. It is the clock of the heavens. A year is Earth's orbital period; a month traces back to the Moon's. Ask how long a trip around takes, and you are asking for the period.

For a circular orbit the period is the distance around, 2 pi r, divided by the orbital speed v: T = 2 pi r / v. Substituting v = sqrt(G M / r) gives the tidy result T = 2 pi sqrt(r^3 / G M), where M is the central mass. This is Kepler's third law in its exact form, T^2 = (4 pi^2 / G M) r^3, showing that period grows steeply as the orbit widens. Crucially, the period depends only on the size of the orbit and the central mass, not on the mass of the orbiting object, so a bolt and a bus at the same altitude keep perfect pace.

You meet orbital periods everywhere in space operations. The ISS circles Earth in about 90 minutes, so astronauts see roughly sixteen sunrises a day. The Moon's period is about 27.3 days. A geostationary communications satellite is deliberately placed at the exact radius where its period equals one day, so it hangs over the same spot on the equator.