The Collaborative International Dictionary of English v.0.48:

Isopycnic \I`so*pyc"nic\, n. (Physics)
   A line or surface passing through those points in a medium,
   at which the density is the same.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Isopycnic \I`so*pyc"nic\, a. [Iso- + Gr. pykno`s dense.]
   (Physics)
   Having equal density, as different regions of a medium;
   passing through points at which the density is equal; as, an
   isopycnic line or surface.
   [1913 Webster]