The term anisotropic denotes a substance of which the properties vary depending on the direction; for example, in crystal optics, in an anisotropic (or birefringent) crystal the speed of light depends on the direction of propagation. Viewed using an optical microscope this phenomenon results in colours appearing.
Most of the time composite materials are anisotropic unlike metallic materials which are isotropic.
For example, in a unidirectional sheet of material, the strength and rigidity are much greater in the direction of the fibres than in the other directions.
It is essential to take anisotropy into consideration when designing and sizing a composite material structure since its behaviour will be very different from that of an isotropic structure with the same geometry.
Anisotropy makes it possible to arrange materials depending on the load at each point. This makes it possible to significantly reduce the quantity of material and hence the weight, a vital criterion in aeronautics and in the space sector.