Antioxidant
Antioxidants are nutrients and other substances that protect cells in the body from the damage caused by "oxygen free radicals" (molecules that seek to become oxidized, a process that harms body tissues and has been linked to many diseases, including stroke, heart disease, and cancer).

Arrhythmia
Arrhythmia is a general term for an irregular heartbeat, caused by problems in the heart's electrical system, which regulates heart rhythm.  A heartbeat can be too slow (bradycardia), too fast (tachycardia), or simply irregular.

Atrial fibrillation
Atrial fibrillation (AF) is a type of arrhythmia in which the upper chambers of the heart (the atria) quiver instead of contracting normally. Therefore, the heartbeat does not pump blood through the heart effectively. 

Automated external defibrillators (AED)
An AED is a small, portable device, attached to a person's chest with wires, that checks the person's heart rhythm, decides if that rhythm is "off," and gives the heart an electric shock (called a defibrillating shock) that restores the correct, natural rhythm. 

Beta-blockers
Beta-blockers are medicines that reduce the heart's tendency to beat faster.

Bradycardia
Bradycardia is a type of arrhythmia in which the heart beats more slowly than it should.

Cardiopulmonary resuscitation (CPR)
CPR is a series of emergency steps given to people whose hearts have stopped pumping blood (a condition known as cardiac arrest).  These steps include rescue breathing (mouth-to-mouth resuscitation) and chest compressions.  
 
Ejection fraction
The ejection fraction is the amount of blood released from the heart's left ventricle when it contracts.  It's called a "fraction" because it's expressed as a percentage of the total amount of blood in the left ventricle.  For example, an ejection fraction of 80% means that when the left ventricle contracts, 80% of the blood in it is pushed out. 

Implantable Cardioverter-Defibrillator (ICD)
An implantable cardioverter-defibrillator is a tiny, battery operated device that is implanted under the skin, near the left collarbone.  It monitors the heartbeat, detects arrhythmias, and sends electrical charges to the heart to restore the correct, natural rhythm.  Because they correct heart rhythms that are too fast, the electrical impulses they generate are usually stronger than those generated by a pacemaker. 

Pacemaker
A pacemaker is a tiny, battery operated device that monitors the heartbeat, detects arrhythmias, and sends tiny electrical charges to the heart to restore the correct, natural rhythm.  Pacemakers are used to correct heart rhythms that are too slow.  They can be external but are more often implanted under the skin.

Tachycardia
Tachycardia is a type of arrhythmia in which the heart beats more rapidly than it should.

Ventricular fibrillation
Ventricular fibrillation is an especially dangerous form of arrhythmia in which the contractions of the ventricles are rapid, unsynchronized, and uncoordinated, so that the heart pumps very little or no blood.