Oxidation is a chemical process that involves the loss of electrons by a molecule, atom, or ion. It occurs when the oxidation state of a molecule, atom, or ion increases, which means that it loses electrons. The term "oxidation" was first used by Antoine Lavoisier to describe the reaction of a substance with oxygen, but it was later realized that the substance loses electrons upon being oxidized, regardless of whether oxygen was involved.

In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to other atoms were fully ionic. It describes the degree of oxidation of an atom in a chemical compound. The increase in the oxidation state of an atom through a chemical reaction is known as oxidation, while a decrease in oxidation state is known as reduction.

There are various definitions of oxidation and reduction in terms of the transfer of oxygen, hydrogen, and electrons/Electrochemistry/Redox_Chemistry/Definitions_of_Oxidation_and_Reduction). Oxidation can be defined as the gain of oxygen, while reduction can be defined as the loss of oxygen/Electrochemistry/Redox_Chemistry/Definitions_of_Oxidation_and_Reduction). Alternatively, oxidation can be defined as the loss of hydrogen, while reduction can be defined as the gain of hydrogen/Electrochemistry/Redox_Chemistry/Definitions_of_Oxidation_and_Reduction). In terms of electron transfer, oxidation is the loss of electrons, while reduction is the gain of electrons/Electrochemistry/Redox_Chemistry/Definitions_of_Oxidation_and_Reduction).

Oxidation and reduction reactions can occur simultaneously in a redox reaction, where the reduced species receives electrons, and the oxidized species loses them. An oxidation process does not need the presence of oxygen, despite its name. Examples of oxidation include the rusting of iron and the reaction between magnesium metal and oxygen to form magnesium oxide.