"Kitty breath" most often refers to an unpleasant odor coming from a cat's mouth. There are several causes for "kitty breath" some of which can be relatively normal for your cat to abnormal and associated with dental or oral abnormalities.
Problems such as inflammation of the gums, abscessed teeth, oral ulcers, or tumors in the mouth can all cause bad breath. Kidney failure and lung tumors can also alter breath odor.
If your cat has bad breath, have him examined by your veterinarian to determine if there are any underlying health problems. Depending on the underlying cause, the cure may be as simple as a dental cleaning or routine brushing of the teeth.
Halitosis is the term used to describe bad breath in cats. For more information, please read Halitosis in Cats