The term is used to compare molecular fragments with each other and with familiar
species from organic chemistry. Two fragments are isolobal if the number, symmetry
properties, approximate energy and shape of the
frontier orbitals and the number of electrons in them are similar.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1129