1. n. 
A solution that could contain more solute than is presently dissolved in it. In brines, an undersaturated solution will not form crystals as easily as if it were saturated or supersaturated. In saltwater muds, an under-saturated fluid is used to allow salt to leach into the mud, keeping the hole from closing in on the drilling assembly. For moisture in air, a relative humidity of less than 100% is under-saturated.