Diffusion is the movement of particles from a high concentration to a low concentration

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Diffusion is the process of movement of particles or molecules of a substance from a region of high concentration to a region of lower concentration.

Suppose there is a gas cylinder in a room and the valve to the outlet is opened, then the gas comes out from a high pressure and high concentration region in side the cylinder and occupies the entire room, which is a lower pressure and lower concentration region.   This is called diffusion.

In other words, diffusion is the property of a gas to spread and occupy all the space available to it.  Perfumes spread in air due to diffusion.

Rate of diffusion is inversely proportional to the square root of density or molar mass of a gas. 

Diffusion property is used in identifying explosive marsh gas ie., methane in mines and in identifying molar mass of unknown gases.