A satellite constellation is a group of satellites operating in coordinated orbital planes to achieve a specific mission objective, typically continuous global or regional coverage. Traditional constellations like GPS (31 satellites in MEO) and Iridium (66 satellites in LEO) were designed with careful orbital mechanics to ensure at least one satellite is always visible from any point on Earth.
Modern constellations have grown dramatically in scale due to falling launch and manufacturing costs. Planet Labs operates 200+ imaging satellites, Spire Global flies 100+ weather and AIS satellites, and HawkEye 360 uses clusters of three satellites for RF geolocation. Constellation design involves tradeoffs between the number of orbital planes, satellites per plane, altitude, and inclination to optimize revisit time and coverage for the mission.