ni·tro·syl \-ˌsil, -ēl\noun (-s) Etymology: International Scientific Vocabulary nitros- + -yl 1.: the nitroso group, radical, or cation — used especially in names of inorganic compounds < nitrosyl chloride > and coordination complexes — compare nitroprusside 2.: a compound of the nitrosyl radical with a metal analogous to a carbonyl < cobalt tricarbonyl nitrosyl Co(CO)3NO >