If you are meaning O2^2- ion, well, don’t forget that ions have a charge, that has to be specified when referring to them. It is a polyatomic ion just because it consists of more than one atom, irrespective of the fact that these atoms are of the same element or not. It was given that name because at the early times chemistry was founded as a science, it was found that with respect to other oxygenated substances, peroxides contained more oxygen than expected: Latin prefix per- gives the terms it is attached to the meaning of being increased, enhanced, and similar.
