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isopycnal

n. A surface of constant density in a fluid (especially in water)

Wikipedia
Isopycnal

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.