A radical or free radical is a species that has one or more unpaired electrons.

eg. 1:  Hydrogen atom
electron configuration of the hydrogen atom:

eg. 2:  Chlorine atom
electron configuration of the chlorine atom:

eg. 3:   Oxygen molecule
electron configuration of the oxygen molecule:

eg. 4:

eg. 5:

Some radicals are resonance-stabilized.

eg:

A radical that has two unpaired electrons is called a diradical (eg. 3).

see also alkyl radical, allylic radical, benzylic radical