Wiktionary
n. (context physics English) the average distance travelled between two similar events; especially by molecules in a gas or liquid, electrons or phonons in a crystal or neutrons in a moderator
Wikipedia
In physics, the mean free path is the average distance traveled by a moving particle (such as an atom, a molecule, a photon) between successive impacts (collisions), which modify its direction or energy or other particle properties.
The following table lists some typical values for air at different pressures and at room temperature.
Vacuum range
Pressure in hPa ( mbar)
Pressure in mmHg ( Torr)
Molecules / cm
Molecules / m
Mean free path
Ambient pressure
1013
759.8
2.7 × 10
2.7 × 10
68 nm
Low vacuum
300 – 1
225 – 7.501×10
10 – 10
10 – 10
0.1 – 100 μm
Medium vacuum
1 – 10
7.501×10 – 7.501×10
10 – 10
10 – 10
0.1 – 100 mm
High vacuum
10 – 10
7.501×10 – 7.501×10
10 – 10
10 – 10
10 cm – 1 km
Ultra high vacuum
10 – 10
7.501×10 – 7.501×10
10 – 10
10 – 10
1 km – 10 km
Extremely high vacuum
<10
<7.501×10
<10
<10
>10 km