What is the X in X-mas?
1794, "extremely minute particle," from French molécule (1670s), from Modern Latin molecula, diminutive of Latin moles "mass, barrier" (see mole (3)). A vague meaning at first; the vogue for the word (used until late 18c. only in Latin form) can be traced to the philosophy of Descartes. First used of Modern Latin molecula in modern scientific sense by Amedeo Avogadro (1811).
molecule mol·e·cule (mŏl'ĭ-kyōōl')
The smallest particle into which an element or a compound can be divided without changing its chemical and physical properties; a group of atoms that is held together chemically.