Anisotropic equations Someone was giving a talk about modeling tumor growth in 3D, after which someone asked the question: "Are all of your equations anisotropic?" It sounded like he was referring to inclusion of unknown parameters. What was he getting at?

Anisotropic means not the same in all directions, not isotropic. With respect to partial differential equations it typically means distinct orders of derivatives wrt space variables. The application you describe might have a simpler interpretation, perhaps stress and strain moduli that vary with spatial direction.