The term 'HEART's rhythm' is referring to the BEAT of the HEART, which is a four letter word: BEAT. The HEART's rhythm is the regular pattern of contractions and relaxations of the HEART muscle that pumps blood throughout the body. This rhythm is controlled by electrical signals that travel through the HEART and can be measured by an electrocardiogram (ECG). The HEART's rhythm is essential for life and any disruption in the rhythm can lead to serious health problems.