Oxidation involves the loss of electrons while reduction is the gaining of electrons. Oxidation-reduction reaction is a reaction which involves the gaining of electrons and loss of electrons.
Oxidation may also be referred to as increase in the oxidation state of an atom while reduction is the decrease of oxidation state of an atom.
The presence of hydrogen gas as a reactant is a best indicator of an oxidation-reduction reaction. This is because in hydrogen gas hydrogen has a zero oxidation number which may increase to show oxidation or may decrease to show reduction after the reaction.
