Anisotropy
The dependence of a physical property on the direction in which it is measured.
Unit
Dimensionless
In depth
Anisotropy arises whenever a structure has different arrangements or bonding along different axes, so it is characteristic of crystalline solids and absent in amorphous ones and in liquids and gases. Refractive index, conductivity, thermal expansion and mechanical strength are all commonly anisotropic.
Examples in the real world
Graphite conducting well within its layers but poorly across them; calcite showing double refraction.