1794, "extremely minute particle," from French molécule (1670s), from Modern Latin molecula, diminutive of Latin moles "mass, barrier" (see mole (n.3)). For ending see -cule. It has 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 ("smallest part into which a substance can be divided without destroying its chemical character") is by Amedeo Avogadro (1811).
unit of molecular quantity, 1902, from German Mol coined 1900 by German chemist Wilhelm Ostwald, short for Molekül (see molecule).
1886, "molecule consisting of several molecules," a sense now obsolete, from macro- + molecule. Apparently coined in "On Macro-molecules, with the Determinations of the Form of Some of Them," by Anglo-Irish physicist G. Johnstone Stoney (1826–1911). Originally of crystals. Meaning "molecule composed of many atoms" is by 1935, from German makromolekul (1922). Related: Macromolecular (by 1931).
word-forming element meaning "across, beyond, through, on the other side of, to go beyond," from Latin trans (prep.) "across, over, beyond," perhaps originally present participle of a verb *trare-, meaning "to cross," from PIE *tra-, variant of root *tere- (2) "cross over, pass through, overcome." In chemical use indicating "a compound in which two characteristic groups are situated on opposite sides of an axis of a molecule" [Flood].