Average way travelled by a gas molecule at a given temperature and pressure between two collisions with other gas molecules. Important for the successful generation of a plasma: When the pressure is too high (free path too short), the electrons and ions cannot develop the speed required for ionisation bombardment; when the pressure is too low (free path too long), the number of ionisation bombardments is too small to maintain the plasma. Furthermore, the mean free path strongly influences the isotropy of etching processes: The higher the pressure, the shorter the free path, the more isotropic (non-directional) the material removal. At atmospheric pressure (1000 mbar), the mean free path of a particle between two collisions is approx. 10-7 m, and (10-3 mbar) 10 cm at the limit to high vacuum.