Question:   What is the difference between ionization and diffusion?

Answer:

Diffusion is a measure of how one thing moves through another over time.  For example, if you were to release a gas into one side of a room, diffusion is the process by which the gas would eventually fill the entire room.

Ionization, on the other hand, is the process by which a neutral atom loses electrons.  For example, adding a bit of energy to a sample of neutral sodium metal will cause the electrons to jump off, forming sodium cations with a charge of +1.