In hydrology and related studies, a cline (from Ancient Greek κλίνειν (klínein) 'to lean') is a comparatively thin, typically horizontal layer within a fluid, in which a property of the fluid varies greatly over a relatively short vertical distance.

Such clines and the respectively varying properties include:

• Chemocline - chemistry^[1]
• Halocline - salinity^[2]
• Lysocline - dissolution of calcite^[3]
• Pycnocline - density^[4]
• Thermocline - temperature^[5]

• Pinti, D.L. (2022). "Chemocline". Encyclopedia of Astrobiology. Springer. p. 1. doi:10.1007/978-3-642-27833-4_5157-3. ISBN 978-3-642-27833-4.
• Emelyanov, Emelyan M. (2005). The Barrier Zones in the Ocean. Springer. ISBN 978-3-540-25391-4.