the Catholic Church is not a denomination. The Protestant churches need a term such as denomination so you can tell them apart. There is "a Catholic Church" there is not "a Protestant church"....there are many thousands of protestant denominations, all of which think that they are "THE" one.
Tell yourself what you want, like I said, I try not to get into Mormon and Catholic church internal teachings for their members, but yes, the Catholics are members of one of the Christian denominations in America.