Diffusion:

Diffusion is the movement of a substance from an area of higher concentration to an area of lower concentration i.e. along a concentration gradient.

OR

Diffusion:

Diffusion is the net movement of molecules or ions from an area of higher concentration to an area of lower concentration i.e. along the concentration gradient.

Explanation:

Since the molecules of any substance (solid, liquid, or gas) are in motion then that substance is about 0 degrees Kelvin or -273 degrees C, energy is available for the movement of molecules from a higher potential state to a lower potential state. In a substance, the majority of the molecules move from higher to lower concentration, although there will be some that move from low to high. The overall movement is thus from high to low concentration. Eventually, if no input in the system the molecules will reach a state of equilibrium where they will be distributed equally throughout the system.

Passive Transport:

Because a cell doesn’t expend energy when molecules diffuse across its membrane, this diffusion is a type of passive transport.

Significance of diffusion:

Diffusion is one principle method of movement of substances within cells as well as the method for essential molecules to cross the cell membrane. Carbon dioxide and Oxygen are among the few simple molecules that can cross the molecules by diffusion. Gas exchange in gills and lungs operates by this process.