Final answer:
Diffusion is the passive transport process by which molecules move from an area of higher concentration to lower concentration across the plasma membrane, eventually reaching equilibrium.
Step-by-step explanation:
The net movement of specific molecules from an area of higher concentration to an area of lower concentration across a plasma membrane is called diffusion. This is a natural process where molecules flow down their concentration gradient, which means they move from a place where they are more concentrated to a place where they are less concentrated until equilibrium is reached.
The process does not require energy and is thus considered a form of passive transport. Simple diffusion allows small non-charged molecules like oxygen and carbon dioxide to move freely through the phospholipid bilayer of the plasma membrane. However, larger or charged molecules use facilitated diffusion with the help of carrier proteins or pore-forming channels to move across the membrane.