**Isopycnal** refers to a surface or a condition in which the density of a fluid remains constant. The term is most often used in oceanography and fluid dynamics to describe layers or surfaces within a fluid (such as seawater) that have the same density, regardless of variations in temperature or salinity. In the ocean, isopycnal surfaces can play a significant role in the movement and mixing of water masses.
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An isopycnal is a surface of constant mass density. In a Boussinesq approximation, it is equivalently a surface of constant buoyancy.