Known as:  Mock Orange, Syringa, Trifoliate Orange
Latin Name:  Philadelphus lewisii (Hydrangeaceae), Poncirus trifoliata
Poisonous Parts:  Fruit.
Symptoms: May produce vomiting, abdominal pain, and in some cases diarrhea.
Treatment: Unless your cat is unconscious or having convulsions, induce vomiting and follow procedures to delay absorption and speed elimination.  Get prompt veterinary assistance.

Image courtesy of Broussea California Flora Photos.