dec·a·me·tho·ni·um \ˌdekəmə̇ˈthōnēəm\noun (-s) Etymology:deca- + methonium : the bivalent substituted ammonium ion [(CH3)3N(CH2)10NCCH3)3]++ derived by methylation of deca-methylene-diamine; also: any salt containing this ion (as the iodide or bromide used as a muscle relaxant)