mean free path
Imagine threading your way across a crowded room, taking a few steps before bumping into someone, then a few more before the next collision. A gas molecule lives the same way, careering in a straight line until it strikes another molecule. The mean free path is the average distance it covers between one such collision and the next.
Its length depends on how crowded and how large the molecules are. Pack them in densely, by raising the pressure, and the path shortens because collisions come sooner; make the gas thinner and the path lengthens. Fatter molecules also collide more readily, shortening the path. At ordinary air pressure the mean free path is astonishingly short — far smaller than a speck of dust — even though the molecules themselves move hundreds of metres per second.
The mean free path matters because it governs how a gas transports things — heat, momentum and matter — from place to place. It explains why a gas conducts heat and diffuses the way it does, and why, in a high vacuum where the path grows huge, molecules sail across a chamber without meeting one another at all.
Inside a vacuum tube pumped down low enough, the mean free path grows longer than the tube itself, so an electron or molecule can fly clear from one end to the other without a single collision — the principle behind cathode-ray tubes and electron microscopes.
In high vacuum the mean free path outgrows the container itself.
Do not confuse the mean free path with the distance a molecule actually travels in a straight line as the crow flies. Because the molecule keeps changing direction at every collision, its overall progress across a room is far slower than its raw speed suggests.