A fluid flow is compressible if its density changes appreciably (typically by a few percent) within the domain of interest. Typically, this will occur when the fluid velocity exceeds Mach 0.3. Hence, low velocity flows (both gas and liquids) behave incompressibly.

Capable of being pressed together or forced into a narrower compass, as an elastic or spongy substance.

capable of being compressed or made more compact; "compressible packing materials"; "a compressible box"

capable of being easily compressed

see Tableting

Having a description that is smaller than itself; not random; possessing regularity.

Having a description that is smaller than the object itself.

Excessive decrease in volume of soft soil under load.