Final answer:
The T-sign refers to the bright horizontal echo seen on an ECG that indicates the presence of an atrial or ventricular septal defect.
Step-by-step explanation:
The bright horizontal echo that gives the edge of an atrial or ventricular septal defect a broadened appearance is referred to as the T-sign. This finding is seen on an electrocardiogram (ECG) and indicates the presence of a septal defect, a hole in the wall between the chambers of the heart. The T-sign is a valuable diagnostic clue in identifying these types of defects.