Dephosphorylation is the removal of a phosphate (PO43-) group from an organic compound by hydrolysis. It is a reversible post-translational modification that is coupled to the addition of phosphate groups, or phosphorylation. A highly regulated process, dephosphorylation activates and deactivates enzymes by cleaving phosphoric esters and anhydrides. A notable occurrence of dephosphorylation is the conversion of ATP to ADP and inorganic phosphate. Dephosphorylation employs a type of hydrolytic enzyme, or hydrolase, which cleave ester bonds. The prominent hydrolase subclass used in dephosphorylation is phosphatase. Phosphatase removes phosphate groups by hydrolysing phosphoric acid monoesters into a phosphate ion and a molecule with a free hydroxyl (-OH) group. The reversible phosphorylation-dephosphorylation reaction occurs in every physiological process, making proper function of protein phosphases necessary for organism viability. Because protein dephosphorylation is a key process involved in cell signalling, protein phosphatases are implicated in conditions such as cardiac disease, diabetes, and Alzheimer's disease.

Reference
Wiki: Dephosphorylation