What do you call a reaction where a compound loses a water molecule?

A reaction in which a compound loses a water molecule is referred to as a dehydration reaction. This type of reaction involves the removal of water, typically resulting in the formation of a double bond or a more complex molecule from simpler reactants. Dehydration reactions are common in the formation of esters and polysaccharides, where smaller molecules (like alcohols or sugars) combine to form larger compounds with the loss of water.

While hydration reaction refers to the addition of water to a compound, hydrolysis involves breaking down a compound by the addition of water. Condensation reactions, while similar to dehydration reactions in that they often involve the joining of two molecules, typically denote the formation of a larger molecule with the elimination of a small molecule, which is often water but can also include other smaller molecules. Thus, the correct term for the specific reaction where a compound loses a water molecule is indeed a dehydration reaction.