Abbreviation for Highest Occupied 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 highest energy that contains electrons is called the Highest Occupied Molecular Orbital.
The HOMO can reflect the stability of a molecule, especially when compared to the LUMO. The difference in energy between the HOMO and LUMO is known as the gap.