The term "isopycnic" is commonly encountered in the fluid dynamics of compressible fluids, such as in meteorology and geophysical fluid dynamics, astrophysics, or the fluid dynamics of explosions or high Mach number flows. It may also be applied to other situations where a continuous medium has smoothly-varying density, such as in the case of an inhomogeneous colloidal suspension.
Compare also with isochoric. Note that one may discuss isobaric surfaces or isobaric processes; likewise one may discuss isothermal surfaces or isothermal processes. However, the use of "isopycnic" is typically reserved for surfaces and not processes. In particular, unless there is a flux of mass into or out of a control volume, a process which occurs at a constant density also occurs at a constant volume and is called an isochoric process and not an isopycnic process.
The term Isopycnic is also encountered in biophysical chemistry and usually in reference to a process of separating particles, sub cellular organelles, or other substances on the basis of their density. Isopycnic centrifugation refers to a method wherein a density gradient is either pre-formed or forms during high speed centrifugation, after this gradient is formed particles move within the gradient to the position having a density matching their own (this is in fact an incorrect description of the exact physical process but does describe the result in a meaningful way). This technique is extremely powerful.