1. The effective osmotic pressure of a solution relative to another solution from which it is separated by a membrane permeable only to water molecules. It is determined by the concentration of nonpenetrating solutes in the solution relative to that in the other solution. Hence, the tonicity of a solution surrounding a cell determines whether the solution will cause water to flow in or out of the cell across the plasma membrane. A hypertonic solution will cause water to flow out of the cell, whereas a hypotonic solution will cause water to flow into the cell. If the solution is isotonic, there will be no net flow of water across the cell membrane. See osmosis.

2. The degree of muscle tone.