mean free path
How far, on average, a molecule travels before it bumps into another one. In a crowded room you take short steps between collisions; in an open field you can go a long way. The mean free path is that average between-collision distance for a gas molecule.
Precisely, lambda = 1 / (sqrt(2) pi d^2 n), where d is the molecular diameter and n = N/V is the number density (molecules per unit volume). Bigger molecules or a denser gas make the path shorter. For air at room conditions lambda is about 70 nanometres, hundreds of molecular diameters, yet each molecule still suffers billions of collisions every second.
Use: the mean free path governs how gases spread and share energy, underlying viscosity, diffusion, and heat conduction. In a high vacuum lambda can grow larger than the container itself, so molecules hit the walls more often than they hit each other, which matters for vacuum technology and for spacecraft.
Roughly 70 nanometres in air at normal pressure, but metres or more in a high vacuum where the gas is extremely thin.
The average distance travelled between collisions.
The mean free path shrinks as the gas gets denser; that is why a molecule at atmospheric pressure travels only tiny hops even though it moves at hundreds of metres per second.