Wiktionary
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.