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During a redox reaction, electrons are transferred between two species, which is the defining characteristic of these reactions. The term "redox" itself is derived from the conjunction of "reduction" and "oxidation." In any redox process, one species undergoes oxidation by losing electrons, while another species undergoes reduction by gaining those electrons.

This transfer of electrons leads to changes in the oxidation states of the elements involved, which is a critical aspect of understanding how elements interact in chemical reactions. For instance, in reactions involving metals and non-metals, the metal may lose electrons to form cations, while a non-metal accepts these electrons to form anions. These electron transfers are fundamental to various processes, such as combustion, respiration, and corrosion, showcasing the importance of redox reactions in both chemistry and everyday life.

In contrast to the other options, they describe different types of chemical processes that do not involve the key aspect of electron transfer. Proton exchange is characteristic of acid-base reactions, decomposition reactions involve breaking down compounds into simpler substances, and bond formation among different elements does not necessarily imply a transfer of electrons as seen in redox reactions.