An electrically neutral entity consisting of more than one atom
(

).
Rigorously, a molecule, in which

must correspond to a depression on the potential energy surface that is deep enough
to confine at least one vibrational state.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1143