Final answer:
Oxidation involves the loss of valence electrons by an atom, while reduction is the gain of valence electrons. These processes always occur together in redox reactions, as one atom's loss is another atom's gain.
Step-by-step explanation:
The terms oxidation and reduction refer to the processes involving the transfer of valence electrons between atoms in chemical reactions. Oxidation is the process whereby an atom loses valence electrons, which can also be associated with the gain of oxygen or loss of hydrogen, as well as an increase in oxidation number.
Conversely, reduction involves the gain of valence electrons, which can be accompanied by the loss of oxygen or the gain of hydrogen, or a decrease in oxidation number.
Chemical reactions that involve these electron transfers are known as oxidation-reduction reactions, or simply redox reactions.
It is important to note that oxidation and reduction always occur simultaneously, as one substance loses electrons while another gains them. This concept is often summarized by the mnemonic OIL RIG, where 'Oxidation Is Loss' and 'Reduction Is Gain' of electrons.