Escape Velocity :

Escape velocity, in astronomy and space exploration, is the velocity that is sufficient for a body to escape from a gravitational centre of attraction without undergoing any further acceleration. Escape velocity decreases with altitude and is equal to the square root of 2 (or about 1.414) times the velocity necessary to maintain a circular orbit at the same altitude. At the surface of the Earth, if atmospheric resistance could be disregarded, escape velocity would be about 11.2 km (6.96 miles) per second. The velocity of escape from the less massive Moon is about 2.4 km per second at its surface. A planet (or satellite) cannot long retain an atmosphere if the planet's escape velocity is low enough to be near the average velocity of the gas molecules making up the atmosphere.

Excerpt from the Encyclopedia Britannica without permission.