A material is isotropic if its mechanical properties are the same in all directions. Isotropic materials can have a homogeneous or non-homogeneous microscopic structure. For example, steel demonstrates isotropic behavior although its microscopic structure is non-homogeneous.
The elastic properties of an isotropic material are defined by the modulus of elasticity (EX) and Poisson's Ratio (NUXY). If you do not define a value for NUXY, the program assumes a value of 0.