A one-electron function used in the expansion of the molecular orbital function. Basis functions are commonly represented by atomic orbitals (see also Slater-type orbital or Gaussian-type orbital) centered on each atom of the molecule.
Source:
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1925 (https://doi.org/10.1351/pac199971101919)