Do not confuse this prefix with peri-. (1) Through, conveying intensity. (2) In chemistry, a prefix denoting either 1) more or most, with respect to the amount of a given element (usually oxygen, as in perchloric acid) or radical contained in a compound, or 2) the degree of substitution for hydrogen, as in peroxides, peroxy acids (e.g., hydrogen peroxide, peroxyformic acid).peroxy-. [L. through, throughout, extremely]