Plain English Meaning

It's the pull that keeps the tiny particles of a substance stuck to each other. In air, that pull is very small, so air spreads out and moves around easily instead of holding together like water or metal.

Full Definition

The attractive force between molecules of the same substance that holds them together. In the atmosphere, this force between air molecules is very weak, which is why air flows freely, mixes easily, and can be compressed.

Context Anchor

Seen in discussions of the structure of the atmosphere, air pressure, density, and why air can support flight.

Derivation

From Latin 'cohaerere,' meaning 'to stick together' ('co-' = together, 'haerere' = to cling). 'Molecular' refers to molecules — the smallest pieces of a substance. So the term literally means 'molecules sticking together,' which is exactly what it describes.