In chemistry, the non-systematic* word or phrase used to describe a molecule. eg `acetone' rather than `propan-2-one' for (CH3)2C=O
* the chemical nomenclature system has been standardised by the IUPAC (International Union of Pure and Applied Chemistry), and gives rise to ridiculous things like `2,8-dibromodibenzobicyclo[b,f][3.3.1]nona-5a,6a-dien-6,12-dione'