The Collaborative International Dictionary of English v.0.48:

Nitrosyl \Ni*tro"syl\, n. [Nitroso- + -yl.] (Chem.)
   The radical -NO, called also the nitroso group. The term
   is sometimes loosely used to designate certain nitro
   compounds; as, nitrosyl sulphuric acid. Used also
   adjectively.
   [1913 Webster]