Jeans escape; escape parameter কী?

Jeans escape is the loss to space of the particles of the Maxwell–Boltzmann tail that move upwards at the exobase faster than the escape velocity. The escape parameter of particles of mass $m$ at the exobase, of radius $r$ and temperature $T$, on a planet of mass $M$, is

the ratio of their gravitational binding energy to $kT$, and also the square of the escape velocity over the most probable thermal speed $u$.