Cardiology. Record of the electrical activity of the heart. For analysis of the ECG the record is subdivided in waves, intervals and segments. Characteristic deflections are P wave, QRS complex, and T wave. Interval refers to the length of a wave plus the isoelectric line that follows it up to the next wave and named by using the letters of both waves on either side (e.g., QT interval). Segment refers to the isoelectric line between the end of one wave and the beginning of the next wave (e.g., ST segment).