Atrium

Atrium refers to a space. In human anatomy, the most common use for the word atrium is in the heart. Our hearts consist of 4 chambers, two of which are known as atriums. The other two chambers are known as ventricles.

The right atrium receives de-oxygenated blood, which it passes to the right ventricle. This blood is then sent to the lungs, where it is infused with oxygen and is delivered back to the heart via the left atrium. It passes from here onto the left ventricle, where it is forcefully ejected to pass around the body.
Simply put, the atria are the smaller of the heart chambers.