An amphoteric oxide is an oxide that can act both as an acid and as a base. An example is aluminium oxide, Al2O3. It reacts with acids to form the corresponding aluminium salt, and with bases to form salts called aluminates.
Al2O3 + 6HCl → 2AlCl3 + 3H2O
Al2O3 + 2NaOH → Na2Al2O4 + H2O