Pressure-gradient parameter of a protoplanetary disk ($\eta =-\frac{1}{2}(c_s/v_K{)}^{2}d\ln P_g/d\ln r$)

A convention for the small fractional reduction of azimuthal gas speed due to radial pressure support. With $c_s^2=P_g/{\rho }_g$, radial balance gives $v_{g\varphi }^2=v_K^2(1-2\eta )$ and hence $v_{g\varphi }\simeq v_K(1-\eta )$. Other authors absorb the factor of two into the definition; the convention must accompany a drift formula.