What type of reaction is characterized by the transfer of electrons?

Redox reactions, short for reduction-oxidation reactions, are specifically characterized by the transfer of electrons between two substances. In these reactions, one substance undergoes oxidation, which involves the loss of electrons, while another undergoes reduction, involving the gain of electrons. This electron transfer is crucial because it accompanies changes in oxidation states of the reactants involved.

In contrast, combustion reactions involve the rapid reaction of a substance with oxygen, often producing heat and light, but they do not primarily focus on the exchange of electrons in the same way as redox reactions. Precipitation reactions, on the other hand, involve the formation of solid substances from solutions but also do not involve the transfer of electrons directly. Synthesis reactions involve the combination of reactants to form a single product, which may not always involve any electron transfer processes fundamentally.

Thus, the defining feature of redox reactions is the electron transfer mechanism, distinguishing them from the other types of reactions listed.