Specific heat (specific heat capacity) is the ratio of increased temperature to added heat for a substance. This ratio depends upon pressure and volume, so for gases, the quoted values generally specify it to be one of two particular cases: as the specific heat of gas kept at a constant volume (CV) or kept at a constant pressure (CP).
The ratio of these (CV/CP) is known as the adiabatic index (γ). An index of 5/3 is well-known as a reasonable index for monatomic gases, a common state within stars. It is used in simple stellar structure models to determine whether convection is taking place (mixing length theory) and can be used in polytrope equations of state in modeling convective stars.
Solids, which do not fill a volume in the manner of gases, generally have one cited specific heat capacity.