Diffusion is the movement of a substance across a membrane from a level of high concentration to the level of low concentration, and can be with gas or liquid.
All throughout the plant, diffusion is taking place.
Gaseous diffusion/exchange - carbon dioxide and oxygen via stomata
Liquid diffusion: water moving from root hair to the xylem
Transpiration - water loss through leaves.
Osmosis is the diffusion of water across a semi-permeable membrane from a solution that is less concentrated to one that is more concentrated.
This happens throughout the plant too. Some examples are:
Water uptake from the soil and into the root hairs
Water uptake into cells, creating cell expansion and turgidity