In gas dynamics, the Landau derivative or fundamental derivative of gas dynamics, named after Lev Landau who introduced it in 1942, refers to a dimensionless physical quantity characterizing the curvature of the isentrope drawn on the specific volume versus pressure plane. Specifically, the Landau derivative is a second derivative of specific volume with respect to pressure. The derivative is denoted commonly using the symbol ${\displaystyle \Gamma }$ or ${\displaystyle \alpha }$ and is defined by

$${\displaystyle \Gamma ={\frac {c^{4}}{2\upsilon ^{3}}}\left({\frac {\partial ^{2}\upsilon }{\partial p^{2}}}\right)_{s}}$$

where

Alternate representations of ${\displaystyle \Gamma }$ include

$${\displaystyle {\begin{aligned}\Gamma &={\frac {\upsilon ^{3}}{2c^{2}}}\left({\frac {\partial ^{2}p}{\partial \upsilon ^{2}}}\right)_{s}={\frac {1}{c}}\left({\frac {\partial \rho c}{\partial \rho }}\right)_{s}=1+{\frac {c}{\upsilon }}\left({\frac {\partial c}{\partial p}}\right)_{s}\\[2ex]&=1+{\frac {c}{\upsilon }}\left({\frac {\partial c}{\partial p}}\right)_{T}+{\frac {cT}{\upsilon c_{p}}}\left({\frac {\partial \upsilon }{\partial T}}\right)_{p}\left({\frac {\partial c}{\partial T}}\right)_{p}.\end{aligned}}}$$

For most common gases, ${\displaystyle \Gamma >0}$, whereas abnormal substances such as the BZT fluids exhibit ${\displaystyle \Gamma <0}$. In an isentropic process, the sound speed increases with pressure when ${\displaystyle \Gamma >1}$; this is the case for ideal gases. Specifically for polytropic gases (ideal gas with constant specific heats), the Landau derivative is a constant and given by

$${\displaystyle \Gamma ={\tfrac {1}{2}}(\gamma +1),}$$

where ${\displaystyle \gamma >1}$ is the specific heat ratio. Some non-ideal gases falls in the range ${\displaystyle 0<\Gamma <1}$, for which the sound speed decreases with pressure during an isentropic transformation.