Dehydration reaction

Dehydration is the general term for a chemical reaction in which a molecule of water ($\mathrm{H}_2\mathrm{O}$) is removed. A common biological instance of a dehydration reaction occurs when a hydroxyl group ($\mathrm{OH}$) belonging to one molecule splits off with a hydrogen atom ($\mathrm{H}$) from a related molecule, which are united to form the water molecule. Typically, the two original molecules then bond, and the repetition of this process creates biopolymers like nucleic acids and proteins.

The opposite reaction to dehydration is called, fittingly, a hydration reaction; if the result of hydration is to split a large molecule into two components (e.g., when breaking a polymer into monomers), then the specific type of reaction is known as hydrolysis.