Definition
The natural mixing of two or more gases or liquids caused by the random motion of their molecules, which spread out until they are evenly distributed throughout the available space.
Plain English
When two gases or liquids are next to each other, their molecules naturally drift around and mix together until the mixture is even, without any stirring or pumping needed.
Context Anchor
Seen in discussions of engine inlets, turbine-engine airflow, and expanding air passages.
Derivation
From the Latin diffundere, meaning 'to pour out' or 'spread abroad' (dis- 'apart' + fundere 'to pour'). The image is of something pouring out and spreading evenly on its own — which is exactly what gas and liquid molecules do.
Why Pilots Care
Diffusion is how fuel vapor mixes with air in a carburetor or fuel-injection system, and how oxygen mixes with cabin air in pressurized aircraft. Understanding it helps explain why mixtures form evenly without mechanical stirring.
Analogy
Like a drop of food coloring placed in a glass of still water — given enough time, it spreads through the whole glass on its own.
Grounding Statement
Picture fast air entering a gently widening passage: as the space opens up, the air slows and its pressure increases.
Intuition Check
Diffusion does not just mean air spreading randomly. In this aviation use, it means a designed slowing of airflow to raise pressure.
Example Sentence 1
Inside the carburetor, diffusion helps the vaporized fuel mix evenly with the incoming air before it reaches the cylinders.
Example Sentence 2
Stable air limits diffusion, so moisture stays concentrated near the surface and keeps ceilings low.