A combination (synthesis) reaction is defined as:

A combination (synthesis) reaction is characterized by the process of combining two or more reactants to produce a single product. This type of reaction involves elements or compounds coming together to form a more complex substance. For instance, if hydrogen gas reacts with oxygen gas, they can combine to form water (H₂O).

This definition differentiates combination reactions from other types of chemical reactions. In contrast, a reaction that starts with one reactant to form multiple products describes a decomposition reaction, where a single compound breaks down into simpler substances. Meanwhile, the process of one element replacing another in a compound depicts a single replacement reaction. Lastly, breaking down a substance into simpler components aligns with the definition of a decomposition reaction as well. Thus, the definition of a combination reaction clearly establishes that it involves multiple reactants uniting to create one unique product.