pen·tyl \ˈpentə̇l\noun (-s) Etymology: International Scientific Vocabulary penta- + -yl : any of eight isomeric alkyl radicals C5H11 derived from the three pentanes by removal of one hydrogen atom: as a.: the normal radical CH3(CH2)3CH2− — called also amyl, n-amyl, n-pentyl b.: isopentyl c.: neopentyl