What does reduction mean in a redox reaction?

In a redox reaction, reduction specifically refers to the gain of electrons by a substance. When a species gains electrons, its oxidation state decreases, which is a defining characteristic of reduction. This electron transfer is crucial in redox reactions because it occurs alongside oxidation, where another substance loses electrons.

Understanding this concept is fundamental in chemistry, as many chemical reactions involve changes in electron distribution. The term "reduction" may also imply a decrease in oxidation state, underscoring the connection between electron gain and the overall transformation of the substances involved.

Other options do not accurately describe the concept of reduction. The loss of protons would typically be unrelated to redox reactions; variations in molecular weight do not inherently signify reduction, and converting a gas to a solid pertains more to physical state changes rather than an electron transfer process. Thus, the correct definition of reduction in the context of redox reactions is indeed the gain of electrons by a substance.