A<unk>  in organic chemistry is a molecular species with two electrons occupying two degenerate molecular orbitals (MO) of the same energy  . They are known by their higher reactivities and shorter lifetimes. In a broader definition<unk> s are even-electron molecules that have one bond less than the number permitted by the standard rules of valence  . The electrons can pair up with opposite spin in one MO leaving the other empty. This is called a singlet state.  Alternatively each electron can occupy one MO with spins parallel to each other. This is called a triplet state. The related radical has just one free electron. The phrases singlet and triplet are derived from the appearance of<unk> s in electron spin resonance: a singlet<unk>  displays a single peak in its spectrum and a triplet has its peak split into a central peak with two adjacent peaks.
The triplet state has total spin quantum number S = 1  and is paramagnetic . The singlet state has S = 0 and is diamagnetic. The degeneracy of each state can be found with Hund's rule of maximum multiplicity: 2S + 1.
In molecules the free electrons can reside on one atom or on different atoms. A molecule can have a singlet state or triplet state with different energy and both states can inter-convert by a process called intersystem crossing. Phosphorescence is based on this principle.
Discrete molecules with a<unk>  nature are singlet oxygen and triplet oxygen. Other important<unk> s are carbenes and nitrenes. Lesser known<unk> s are nitrenium ions and organic so-called non-Kekulé molecules in which the electrons reside on different carbon atoms.
Diradical
Complex 
Cyclic compound
Heterocyclic compound
Bifunctional catalyst
Biopolymer
Liquid
Covalent bond