What is an abnormal heart rhythm that makes the heart unable to pump blood effectively?

We usually refer to these terms as either arrhythmia or dysrhythmias. Under most normal circumstance however, Dysrhythmias are the term I hear most thrown around, but both are the same. Two very dangerous and common dysrhythmia's which are responsible for what we call "Cardiac Arrest" include V. Fib (Ventricular Fibrillation) and PVT/VT (Pulseless Ventricular Tachycardia/Ventricular Tachycardia).

Arrhythmia is probably the term you're looking for.

The prefix "a-" means "not" and the suffix/root "-rhythmia" means "rhythmic."