It is the amount of heat per molecule of the body.
"A Text-Book of Astronomy"
George C. Comstock
But upon the moon, or any other body where the force of gravity is small, conditions are quite different, and in our satellite a velocity of little more than one mile per second would suffice to carry a molecule away from the outer limits of its atmosphere.
"A Text-Book of Astronomy"
George C. Comstock
This motion is only pursued for a short distance in one direction before a molecule comes into collision with some other molecule, whereby the directions and velocities of the individual molecules are continually changed.
"The Story of the Heavens"
Robert Stawell Ball