A chemical reaction in which two or more substances combine to form a single substance

The answer is: Union reaction.

A chemical reaction in which two or more substances combine to form a single substance is known as a formation reaction. This is a reaction where the reactants combine and form a new product with a different chemical structure. In this reaction, the molecules of the reactants are rearranged to form the new product. This reaction is a common occurrence in nature, as it is the basis of many natural processes. It can also be induced artificially in a laboratory setting. Examples of this type of reaction include combustion reactions, dissociation reactions, and substitution reactions. Combustion reactions involve a mixture of oxygen and hydrocarbons to produce carbon dioxide, water, and energy. Dissociation reactions involve the separation of molecules through the use of heat or light, while replacement reactions involve the replacement of one atom of one element by another.