per ‧ ox ‧ ide /pəˈrɒksaɪd $ -ˈrɑːk-/ BrE AmE noun [uncountable]
[ Date: 1800-1900 ; Origin: per- (from Latin , from per ; ⇨ ↑ per ) + oxide ]
a liquid chemical used to make hair light in colour or to kill ↑ bacteria :
her waist-length peroxide blonde hair (=hair made light yellow using peroxide)