An antibonding molecular orbital is a type of chemical bond given by the overlap of two half-full molecular orbitals. This kind of bond weakens the chemical bond between two atoms and helps to raise the energy of the molecule relative to the separated atoms. Such an orbital has one or more nodes in the bonding region between the nuclei.
A molecular-bound orbital becomes an anti-bonded orbital when the electron density between the two nuclei is less than what would be if the two nuclei were separated. Antibonding orbitals are labeled with the asterisk (*) in molecular orbital diagrams; they derive from the out-of-phase superposition of wave functions and are characterized by greater energy than the bound orbitals.
In the case of the diatomic hydrogen (H2) molecule, each atom contributes to the orbital with a single electron, therefore only the orbital σ is occupied and the molecule is more stable than the two separate atoms that compose it.