An approximation to the dynamical equations of motion whereby density is assumed to be constant except in the buoyancy term, -g ho', of the vertical velocity equation. The approximation is reasonable if the vertical extent of the dynamics being considered is much smaller than the density scale height - the height over which the density changes by a factor e. It is generally applicable to most oceanographic circumstances. If a system does not satisfy the Boussinesq approximation it is said to be non-Boussinesq.