Low Earth Orbit
Low earth orbit or LEO refers to the lowest orbits that are stable for a reasonable length of time (months?). Typically around 200 - 1200 km (124 - 726 miles) above the Earth's surface. These orbits tend to have orbital velocities around 27,400 km/h (8 km/s), making one revolution in about 90 minutes but are only a small distance above the Earth compared to other orbits such as geostationary orbits. (See: GEO)
LEO is used as a baseline to compare the payload capacity of rockets.