From Glossary of Meteorology
The altitude (and its corresponding pressure Psat) to which an air parcel must be lifted dry-adiabatically or lowered moist-adiabatically to be just saturated (100% relative humidity with no liquid water present).
For unsaturated air, this is commonly known as the lifting condensation level. Saturation level is a conserved variable that does not change during adiabatic lifting or lowering of saturated or unsaturated air and can thus be used as a tracer for that air parcel. When paired with the corresponding saturation air temperature at that altitude, the result is a saturation point that can be represented on a thermodynamic diagram.