Illustrated Glossary of Organic Chemistry

Diradical: An ion or molecule with two unpaired electrons, usually in two degenerate molecular orbitals.

Double bond oxygen:
Lewis theory gets it wrong.
Diradical oxygen:
Quantum mechanics gets it right.
A simple interpretation of certain properties of molecular oxygen (O2) suggest this molecule is a diradical, with an oxygen-oxygen bond order = 1 (i.e., a single bond) and one unpaired electron on each oxygen atom. The molecule does not have an oxygen-oxygen double bond as predicted by simple Lewis structure theory.