When someone is having an irregular heartbeat, doctors will sometimes use a procedure called defibrillation to shock the patient's heart back into a normal rhythm. You've probably seen defibrillation in TV shows and movies.
Defibrillation was invented in the 1890s by Swiss scientists, but it wasn't used on human patients until the 1930s. The word means "stopping of fibrillation," which is the medical term for when a part of the body twitches and quivers irregularly. When the heart does this, it's called ventricular fibrillation. External defibrillation — involving the paddles that EMTs and ER doctors use today — was developed in the last half of the 20th century.