The type of reaction that forms one large molecule from two or more smaller ones is called a condensation reaction or dehydration synthesis.

Here's why:

* Condensation: This term refers to the formation of a new molecule by the joining of two or more smaller molecules.

* Dehydration Synthesis: This specifically describes the process where a water molecule is removed (dehydrated) from the reactants to form a larger molecule.

Example:

The formation of a disaccharide (like sucrose) from a monosaccharide (like glucose) and a fructose molecule is a classic example of a condensation/dehydration synthesis reaction. A water molecule is released as the two smaller sugars join to form the larger sugar molecule.