In chemistry, a diradical is a molecular species with two electrons occupying molecular orbitals (MOs) which are degenerate.  The term "diradical" is mainly used to describe organic compounds, where most diradicals are extremely reactive and in fact rarely isolated. Diradicals are even-electron molecules but have one fewer bond than the number permitted by the octet rule. 
Examples of diradical species can also be found in coordination chemistry, for example among bis(1,2-dithiolene) metal complexes.