Answer is: oxidation numbers.
In oxido-reduction
reaction, at least one element lose and one element gain electrons.
Oxidation
reaction is increasing of oxidation number of atom, because element lost
electrons in chemical reaction.
Reduction is lowering
oxidation number because atom gain electrons.
For example, in
oxidation-reduction reaction: 2H₂ + O₂ → 2H₂O; hydrogen is
oxidized (change oxidation number from 0 to +1) and oxygen i s reduced (change
oxidation number from 0 to -2).
