If we are talking about irradiance, I understand the following to be true:

a) If irradiance comes from all directions at the same rate, it is said to be isotropic.

b) If irradiance comes from only a single direction, it is said to be anisotropic.

Is it still called anisotropic if I receive photons at the same rate from the left and right (but no other directions)? The key point being that irradiance is not omni-directional, but light pressure causes no net displacement over time as in the isotropic case.

I am just wondering if there's a term for this "half-way" situation.

Your definition b) is wrong. Anisotropic means not isotropic. Radiation from one direction is called monodirectional.

Sorry, I didn't mean to imply that anisotropy denotes unidirectionality, but the other way around.

I'll assume you mean no -- there is no linguistic differentiation between the cases of anisotropy where it's unidirectional or multidirectional.

Thank you.

http://dictionary.reference.com/browse/monodirectional