Final answer:
Systole and diastole usually refer to the heart chambers during contraction and relaxation, respectively.
Step-by-step explanation:
The terms systole and diastole usually appear in reference to the heart chambers. Systole refers to the period of contraction when the heart pumps blood into circulation, while diastole is the period of relaxation when the chambers fill with blood. Both the atria and ventricles undergo systole and diastole to ensure efficient blood flow.