What is the definition of simple diffusion?
Simple diffusion is basically the diffusion of substances through a membrane without needing the help from other substances, such as proteins. An example would be carbon dioxide gas for plants. When the stomata opens, it goes into the plant, as a result of moving from a higher concentration to a region of lower concentration.
The opposite is called facilitated diffusion. This is when molecules needed assistants to help them pass through membranes. An example would be polar molecules. They cannot easily pass through the membrane due to its non-polar lipid layers, and so carrier molecules form a channel/tunnel for them to pass through and into the cell.