Ventricular tachycardia
Ventricular tachycardia (V-tach or VT) is a type of tachycardia (a fast heart rate).  The heart has four chambers (closed spaces).  The top two are called the atria, and the bottom two are called the ventricles.  When the heart is working normally, electrical signals in the heart tell the to atria squeeze first.  However, in ventricular tachycardia, the ventricles squeeze first.  This happens because the electrical signals that tell the atria to squeeze first are not working.