A carbene is a species that has a formally neutral carbon atom bearing two nonbonded valence electrons. A carbene has the following general structural formula.
R1 and R2 could be any ligand
A carbene could exist in two spin states: singlet carbene and triplet carbene. In a singlet carbene, the two nonbonded valence electrons in the electron-deficient carbon are in the same orbital, i.e., they are a lone pair.
In a triplet carbene, the two nonbonded valence electrons in the electron-deficient carbon are in different orbitals. Since a triplet carbene has unpaired electrons, it is, by definition, a radical.
see also carbene equivalent, nitrene