In terms of electron transfer, how can reduction be defined?

Reduction is defined as the process of gaining electrons. This is a fundamental concept in redox (reduction-oxidation) reactions, where two processes occur simultaneously: one substance undergoes oxidation (loses electrons) while the other undergoes reduction (gains electrons). When a substance gains electrons, it often results in a decrease in its oxidation state, indicating a reduction in the charge associated with that atom or ion. This transfer of electrons is crucial in various chemical processes, including those in biological systems, electrochemistry, and energy transfer.

Other processes mentioned in the remaining options do not pertain to reduction. Losing electrons corresponds to oxidation, transferring protons relates to acid-base chemistry rather than reduction, and synthesizing compounds does not inherently involve electron transfer. Therefore, the correct identification of reduction as the gaining of electrons is essential for understanding various chemical reactions and electron flow within those reactions.