Answer
Oxidation is losing electrons and the increase in oxidation number.
Reduction is gaining electrons and the decrease in oxidation number.
Work Step by Step
Always remember "OIL RIG"
OIL Oxidation Is Loss (of electrons)
RIG Reduction Is Gaining (of electrons)
and when a substance loses an electron (which is negative) it becomes more positive, therefore, its oxidation state increases and the opposite is true here for reduction.
