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).