What is osmoregulation?

In simple words, osmoregulation is the control of the water and electrolyte balance in the body.

Osmoregulation ensures the active regulation of the osmotic pressure of bodily fluids to maintain the homeostasis of the body's water content; that is it keeps the body's fluids from becoming too dilute or too concentrated. Organisms in all environments (aquatic and terrestrial) must maintain the right concentration of solutes and amount of water in their body fluids.