Chemical prefix to the name of a compound, indicating: (1) one methyl (CH3) group has been replaced by hydrogen (e.g. noradrenaline can be considered to be a demethylated derivative of adrenaline); (2) an analogue of a compound containing one fewer methylene (CH2) groups than the parent compound; (3) an isomer with an unbranched side-chain (e.g. norleucine, norvaline).