Compressive strength and modulus are tested by placing a specimen between two parallel platens and compressing it to rupture.
Compression strength is the amoun of stress necessary to cause rupture or deformation by a predetermined percentage. Compression modulus, similar to other moduli is the ratio of stress to strain below the proportional limit of the material.
Most engineering thermoplastics have fairly high compression properties which will only rarely place limitations on designs. In most cases tensile or flexural strength impose many more constraints.