What is the X in X-mas?
heterocyclic het·er·o·cy·clic (hět'ə-rō-sī'klĭk, -sĭk'lĭk)
Containing more than one kind of atom joined in a ring.
Of or relating to a compound containing a closed ring structure made of more than one kind of atom. Pyridine (C5H5N), for example, is a heterocyclic compound, as its ring contains five carbon atoms and one nitrogen atom. Compare homocyclic.