The heart pacemaker is a group of cells within the heart that together initiate contractions and set the pace of beating. For most people, this is a group of cells in the right atrium of the heart, otherwise known as the sinoatrial node.
Also related to the heart, "pacemaker" is the common term for an implantable artificial pacemaker device, which was developed by Wilfred Bigelow, John A. Hopps, and J. C. Callahan.