What is the medical term for cardiac arrhythmia?

Cardiac arrhythmia refers to abnormal heart rhythm, as measured with an ECG. A "normal" rhythms is called a "Sinus Rhythm", as it originates in the Sinoatrial node of the heart. Any rhythm that originates elsewhere, or deviates from what is considered "normal" is an arrhythmia. There are many types of arrhythmia. On a cardiac monitor, if we see beats that are abnormal, we call the ectopy, or ectopic beats. But as far as a "term for arrhythmia"... that's the term: Arrhythmia.