In medical terms it's called arrhythmia, which is simply a term that indicates the heart is not beating at a normal rate or rhythm. If your cat has a rapid heartbeat, it could indicate a serious health problem.
Cats have much faster heart rates than people, so don't worry if it seems like your cat's heart is beating twice as fast as yours -- it probably is. The normal heart rate for a healthy adult cat is between 140 and 220 beats per minute. Anything faster than that could signal a problem. To check your cat's heart rate, press your fingers into your cat's front "armpit" or hollow between the front leg and the chest until you feel the heartbeat. Count the number of beats in a 15-second period and multiply it by four to get the total beats per minute.
Keep in mind that a cat that is stressed or has just been running and playing will have a faster heartbeat. Try to check your cat's heart rate when he is resting and relaxed.
An overactive thyroid, or hyperthyroidism, can cause your cat to have a very rapid heartbeat, especially if the disease has gone untreated. In addition to a high heart rate, if your cat has hyperthyroidism he is also likely to have high blood pressure, a noticeable change in appetite, vomiting, and diarrhea. Thyroid disease in cats is linked to heart disease and a fast heartbeat in conjunction with any other symptoms of thyroid problems is cause for a trip to the vet. Once the hyperthyroidism is treated, your cat's heart rate should return to normal.
Cardiomyopathy is the formal term for heart muscle disease. It's also often called heart failure. While it seems backward that the heart would beat faster when it's failing, a cat with heart failure often has a rapid heartbeat. This is because damage to the heart muscle makes it more difficult to keep up a proper amount of blood flow, so the heart tries to work faster to make up for the structural weakness. There is no cure for heart failure and most cats do not live more than a few weeks to a few months after a diagnosis. There are medications, however, that your veterinarian can prescribe to slow down the progress of the disease.
Hyperthyroidism and heart failure are the two diseases most associated with a rapid heartbeat in cats, but don't panic until your cat has seen the veterinarian, since there are plenty of other causes. Nutritional deficiencies, anemia, infections, injuries, hypothermia and fevers can cause an elevated heart rate. Other causes may be kidney disease, poisoning, heartworm, cancer or congenital heart defects. An unusually fast heart rate should be checked out by your veterinarian to determine the cause.