Q.Why imbibition is considered a method of diffusion ?

Drop a little ink into a glass of still water and watch what happens: the colour slowly spreads out on its own until it is evenly mixed, and you never had to stir it. That everyday spreading — from where there is a lot of something to where there is little — is diffusion, and when it happens across a cell's membrane it is called simple diffusion.

In a cell, simple diffusion is one way molecules cross the plasma membrane without the cell spending any energy. The membrane is selectively permeable, meaning it lets some things pass while holding others back. Small, neutral (uncharged) solutes are able to slip straight through the membrane, and they always move down the concentration gradient — that is, from the side where they are more concentrated to the side where they are less concentrated. No pumps, no fuel, no effort from the cell: the molecules simply drift from crowded to less crowded until things even out.

Because it needs no energy, simple diffusion is one form of passive transport. A closely related case is osmosis, which is just the diffusion of water across the membrane, again from where water is more concentrated to where it is less. There is, however, an important limit: polar (charged or water-loving) molecules cannot pass straight through the oily, non-polar interior of the membrane on their own. Such molecules need a carrier protein embedded in the membrane to ferry them across — so they still move down their gradient without energy, but they need a helper to do it. …