|Depictions of molecules: both inanimate (dihydrogen) and animate (retinal molecule, myosin molecule, and human molecule).|
In 1620, Isaac Beeckman, building on Epicurus, outlined the first concept of the molecule. 
In 1649, Pierre Gassendi, having been influenced by Beeckman, in his Syntagma philosophiae Epicuri (Arrangement of the Philosophy of Epicurus), coined the term "molécule" as the attachment of two or more atoms. Gassendi, supposedly, outlined the view that a hooked atom attached to an eyelet atom would form a molecule.  In commentary on evolution, Gassendi stated: 
“While [the atoms] are moving in various ways and meeting, interweaving, intermingling, unrolling, uniting, and being fitted together, molecules or small structures similar to molecules are created, from which the actual seeds are constructed and fashioned.”
The Online Etymology Dictionary, to note, claims, in a mis-attribution sense, that the term molecule dates to 1794 where it was defined as an ‘extremely minute particle’, having originated from a translation of the French term molécule, supposedly first used in 1678, originating in the work of Rene Decartes.  This may be possible; albeit, to note, Gassendi publications and views in support of atomic and molecular theory were spurred into existence so at to contradict the philosophical views of Descartes and it is difficult to find a reference where Descartes actually uses the term molecule.
In his 1808 New System of Chemical Philosophy, English chemist John Dalton was stating that volumes of gas consist of “a number of ultimate particles or molecules”, and using the terms atom and molecule differently. 
It is often said, however, that the first semi-modern scientific use of the term ‘molecule’ is found in the famous 1811 article “Essay on Determining the Relative Masses of the Elementary Molecules of Bodies” by Italian physicist Amedeo Avogadro.  In this paper, Avogadro accepts both John Dalton’s theories and Joseph Gay-Lussac’s data, and shows how to reconcile them by distinguishing between the atom and the molecule of an elementary gas.  It is said that Avogadro and his student Stanislao Cannizzaro coined the term molecule from the Latin name molecula or little mass.  English chemistry historian James Partington summarizes Avogadro’s view as: 
“The smallest particles of gases are not necessarily simple atoms, but are made up of a certain number of these atoms united by attraction to form a single molecule.”
It is to be noted, supposedly, that this is not a literal translation in that Avogadro uses the name "molecule" for both atoms and molecules. Specifically, he uses the name "elementary molecule" when referring to atoms and to complicate the matter also speaks of "compound molecules" and "composite molecules". 
In hmolscience discussions, the query often arises as to whether or not one can classify a person as a "molecule", say as compared to other terms: chemical complex, chemical species, etc. In general terms, the "hall-mark of a molecule", according to English physiologist Charles Sherrington, in his 1938/39 discussions on whether or not a protein is to be regarded as cluster of lesser molecules or a giant molecule or protein molecule, is "definite individuality". 
Further discussion on this subject of molecular classification can be found in German-born American physical organic chemist Ernest Grunwald's 1997 Thermodynamics of Molecular Species, particularly his opening section entitled "Formal Components". In short, according to Grunwald: 
“In general, a molecular species is a macroscopic or near-macroscopic ensemble of molecules that are characterized by a definite molecular formula, a definite and distinctive equilibrium geometry, and a distinctive set of molecular modes of motion and spectral properties. For example, the actors in chemical reaction mechanism—the reactants, products, substrates, catalysts, reactive intermediates, and even the mechanically unstable transition states—all represent separate molecular species, as do any sets of molecules that become distinguishable in physical interaction mechanisms.”
A molecule, in short, is any sets of atoms that become distinguishable in the mechanism interaction process.
In human chemistry, the term "human molecule" is the chemical definition of one human being.
1. Licker, Mark D. (2002). McGraw-Hill Concise Encyclopedia of Chemistry. New York: McGraw-Hill.
2. Molecule – Online Etymology Dictionary.
3. Avogadro, Amedeo. (1811). “Essay on a Manner of Determining the Relative Masses of the Elementary Molecules of Bodies and the Proportions According to Which they Enter Into These Combinations”, Journal de Physiques, 73: 58-76.
4. Dalton, John, Gay-Lussac, Joseph, Avogadro, Amedeo. (1899). Foundations of Molecular Theory: Comprising Papers and Extracts by John Dalton, Joseph-Louis Gay-Lussac, and Amedeo Avogadro (editors: William F. Clay, Marshall Simpkin, and Kent Hamilton) . Edinburg: The Alembic Club.
5. Partington, James. (1937). A Short History of Chemistry. Dover.
6. Carraher, Charles E. and Seymour, Raymond, B. (2003). Giant Molecules: Essential Materials for Everyday Living and Problem Solving (coined the term molecule, pg. 8). Wiley.
8. Dalton, John. (1808). New System of Chemical Philosophy. (extracts, pgs. 70-72). Weale.
9. Gribbin, John. R. (2004). The Scientists: a History of Science told through the Lives of its Greatest Inventors (hook-and-eye molecules, pg. 115). Random House.
10. LoLordo, Antonia. (2007). Pierre Gassendi and the Birth of Modern Philosophy (molecules, 8+ pgs). Cambridge.
11. Carey, George. (1894). The Biochemic System of Medicine (pg. 49). F.A. Luyties.
12. Sherrington, Charles. (1940). Man on His Nature (pg. 109). CUP Archive.
13. Grunwald, Ernest. (1997). Thermodynamics of Molecular Species (pgs. 1-3). John Wiley & Sons, Inc.
14. Kubbinga, Henk. (1988). “The first ‘molecular’ theory (1620): Isaac Beeckman (1588-1637)” (abs), Journal of Molecular Structure, 181(3-4):205-18.
(b) Kubbinga, Henk. (2001). The History of the Concept of the Molecule (Ѻ). Publisher.
● Maxwell, James. (1873). “Molecules” (Ѻ), lecture delivered before the BAAS Bradford; in: Nature, 8: 437-41; in: Phil. Mag. (4):453-69; in: Maxwell’s Scientific Papers, Volume 2 (pg. 361-78); in: Maxwell on Molecules and Gases (editors: Elizabeth Garber and Stephen Brush) (§:16: “Molecules”, pgs. 137-40). MIT Press, 1986.
● Grosberg, Alexander Y. and Khokholv, Alexei R. (2010). Giant Molecules: Here, There, and Everywhere (pg. 1). World Scientific.
● Molecule – Wikipedia.