Higher Dimensions

 If this,

$\cfrac{\partial^2\psi}{\partial\,t^2_c}=ic\cfrac{\partial^2\psi}{\partial\,x\partial\,t_c}$

can accommodate another, $\cfrac{\partial}{\partial\,t}$ or $\cfrac{\partial}{\partial\,x}$, then that is higher dimension.

Nothing else.