Abbreviation for Lowest Unoccupied Molecular Orbital.
According to Molecular Orbital Theory, the atomic orbitals of the component atoms of a molecule combine in linear fashion to make a large number of hybrid electron orbitals. The electrons of all the atoms in the molecule fill these orbitals, starting with the lowest energy orbitals. The molecular orbital with the lowest energy that does not contain electrons is called the Lowest Unoccupied Molecular Orbital.
The LUMO can reflect the stability of a molecule, especially when compared to the HOMO. The difference in energy between the HOMO and LUMO is known as the gap.