Chaplygin gas,^[1][2] which occurs in certain theories of cosmology, is a hypothetical substance that satisfies an exotic equation of state in the form ${\displaystyle p=-A/\rho ^{\alpha }}$, where ${\displaystyle p}$ is the pressure, ${\displaystyle \rho }$ is the density, with ${\displaystyle \alpha =1}$ and ${\displaystyle A}$ a positive constant. The substance is named after Sergey Chaplygin.

In some models, generalized Chaplygin gas is considered, where ${\displaystyle \alpha }$ is a parameter, which can take on values ${\displaystyle 0<\alpha \leq 1}$.