This is pretty much link-only, but anyway...
When searching for official definitions, nomenclature, or recommended procedures, the organisation providing all these is the IUPAC (International Union of Pure and Applied Chemistry)!
An IUPAC book series, known as the Colour Books, provides the normative definitions and guidelines on various topics. These are the results of what chemists in the different committees in the IUPAC divisions have eventually agreed upon.
To my knowledge, all these Colour Books are available online.
As kindly mentioned by Loong
in a comment, the online versions are not necessary the latest ones! The online version of the IUPAC Nomenclature of Organic Chemistry
is from 1993. In addition, a 2004 draft copy
is legally available.
Organic chemists will often refer to the Blue Book (Nomenclature of Organic Chemistry), inorganic chemists to the Red Book (Nomenclature of Inorganic Chemistry), etc.
As far as your question is concerned, the Gold Book (IUPAC Compendium of Chemical Terminology) provides the definitions for molecule and molecular entity.