pycnocline

/P IH1 K N AH0 K L AY0 N/
noun
  1. 1

    A boundary layer in a body of water between areas of different temperature or salinity.

    โ€œThe bay has fresher water at the surface and saltier water at the bottom, in between is the pycnocline.โ€

  2. 2

    A layer of water where the density changes rapidly with depth

Source: https://en.wiktionary.org/wiki/pycnocline