An approximation, useful for the study of equatorial and midlatitude flow, whereby the Coriolis parameter is taken to vary linearly with latitude. Explicitly, the Coriolis parameter is given approximately by f simeq f_0 + eta y in which y is the meridional distance from some fixed latitude where the Coriolis parameter is f_0, and eta (from which the ``beta plane gets its name) is the meridional gradient of f at that fixed latitude.