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 Lane-Emden Equation 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.