2. the gradual difference in the concentration of solutes in a solution between two regions. In biology, a gradient results from an unequal distribution of ions across the cell membrane. When this happens, solutes move along a concentration gradient. This kind of movement is called diffusion.
The movement of solutes along a concentration gradient is common in many biological processes. For a more elaborated discussion, read the Tutorial in Cell Biology where various ways substances pass through a cell membrane are described.