An isopycnal is a line connecting points of a specific density or potential density. Isopycnals are often displayed graphically to help visualize "layers" of water in the ocean or gasses in the atmosphere in a similar manner to how contour lines are used in topographic maps to help visualize topography.
In oceanography, isopycnals are used to display the vertical distribution of water density. In a body of water, as the depth increases, so does the density. Varying degrees of salinity, temperature, and pressure act to modify the density of water, and the denser water always lies below the less dense water. This creates distinguishable layers of water differing physical properties. This phenomenon is called stratification. The strata are held in place by the large differences in physical and chemical properties between layers that prevent mixing. Turbulence can cause upset boundaries between the layers, causing them to bend, which would cause the isopycnals to appear uneven. The ways in which the isopycnals are transformed can be used by oceanographers to identify the force that caused the underwater disturbance.
In Meteorology, isopycnals are used to display different layers of gasses in the atmosphere. In the atmosphere, Varying degrees of humidity, temperature, and pressure change the density of air. Isopycnals are not used in meteorology as frequently as they are in oceanography, since the density gradients observed in the atmosphere are typically gradual, unlike in stratified bodies of water. In these cases, isopycnals are less relevant, since they do not display any substantial features.