Which term refers to the phenomenon where an electron is transferred from one substance to another?

The phenomenon where an electron is transferred from one substance to another is correctly identified as electron transfer. This process plays a fundamental role in various chemical reactions, particularly in redox reactions, where one substance is oxidized (loses electrons) and another is reduced (gains electrons). The transfer of electrons is essential for the movement of charge and the release or absorption of energy in these reactions.

In this context, the term "electron transfer" precisely describes the action of moving an electron from one atom, ion, or molecule to another, which is crucial in many biological processes such as cellular respiration and photosynthesis. This term encompasses the entire process and is not limited to the specific actions described by the other choices. Ionization, for example, refers to the process of gaining or losing electrons to form ions, which is a related but different concept. Oxidation specifically denotes the loss of electrons, while electron affinity relates to the tendency of an atom to attract electrons, rather than the act of transferring them. Thus, electron transfer is the most accurate descriptor for the process specified in the question.