Oxidation is the loss of electrons or increase in oxidation state of a chemical species in a chemical reaction. Oxidation and reduction are two types of chemical reactions that go hand-in-hand to form redox reactions.
Oxidation is a chemical process where an atom, ion, or molecule loses electrons as it interacts with another chemical species, altering its composition and energy state.
Oxidation occurs when an atom, molecule, or ion loses one or more electrons in a chemical reaction. When oxidation occurs, the oxidation state of the chemical species increases. Oxidation doesn't necessarily involve oxygen! Originally, the term was used when oxygen caused electron loss in a reaction. The modern definition is more general.
By definition, the oxidation state of an atom is the charge that atom would carry if the compound were purely ionic. For the active metals in Groups IA and IIA, the difference between the oxidation state of the metal atom and the charge on this atom is small enough to be ignored.
Oxidation is the loss of hydrogen. Reduction is the gain of hydrogen. Notice that these are exactly the opposite of the oxygen definitions (#1). For example, ethanol can be oxidized to ethanal: An oxidizing agent is required to remove the hydrogen from the ethanol.
Oxidation does not mean that an oxygen atom is added to the compound. Instead, it is a chemical reaction that involves the loss of electrons. Metals are generally considered to be elements that can easily lose electrons, so they are known to be easily oxidized.
Oxidation involves removal of an electropositive atom, radical, or electron or, conversely, the addition of an electronegative moiety. These reactions are usually catalyzed by oxygen, heavy metal ions, and light, leading to free radical formation.