(1) Having a lesser degree of tone, tension or tonicity
(2) Having a lesser osmotic pressure in a fluid compared to another fluid
Hypotonic is a descriptive word relating to hypotonicity. In cellular level, hypotonicity may pertain to a property of a solution with a comparatively lower concentration of solutes relative to the amount of solutes in another solution. Also, a solution is described as hypotonic when the other solution being compared with has less osmotic pressure in a fluid compared to another fluid.
Hypotonic may also pertain to a condition or property of a solution that has less tonicity than the other solution with which it is compared. Solutions that have different tonicities will result in a net flow of water across the cell membrane. For example, blood serum that is hypotonic to a physiologic salt solution would result in the net movement of water molecules towards the area with less water concentration (or higher solute concentration).
In the tissue level, a hypotonic muscle would have a lesser degree of tone or tension while the length changes in contrast to a hypertonic muscle that has a greater degree of tone or tension. A muscle whose tension remains relatively constant is described to be isotonic.
Word origin: hypo- (“under”)– + Greek tonos (“tension”) + –ic