What term describes the relaxation, or period of relaxation, of the heart?

The term that describes the relaxation, or period of relaxation, of the heart is diastole. During diastole, the heart's chambers, specifically the ventricles, fill with blood as the heart muscle relaxes. This phase is crucial as it allows the chambers to gather the blood needed for the next contraction.

In contrast, systole refers to the contraction phase where the heart muscle contracts and pumps blood out of the chambers. The other terms mentioned, bradycardia and tachycardia, relate to heart rate. Bradycardia indicates a slower than normal heart rate, while tachycardia refers to a faster than normal heart rate. These terms do not pertain to the specific phases of the heart's contraction and relaxation cycle.