Ground speed refers to the speed at which an aircraft, vehicle, or vessel moves relative to the Earth's surface. It is measured in units such as knots or miles per hour and is important for navigation and flight planning. Ground speed can differ from airspeed, which measures how fast an aircraft moves through the air, due to factors like wind. For example, if a plane is flying into a strong headwind, its ground speed will be lower than its airspeed. Conversely, a tailwind can increase ground speed, allowing the aircraft to reach its destination faster. Understanding ground speed helps pilots and navigators ensure safe and efficient travel.