The rebel yell was a battle cry

Battle cry

A battle cry is a yell or chant taken up in battle, usually by members of the same military unit.Battle cries are not necessarily articulate, although they often aim to invoke patriotic or religious sentiment....

 used by Confederate soldiers

Confederate States Army

The Confederate States Army was the army of the Confederate States of America while the Confederacy existed during the American Civil War. On February 8, 1861, delegates from the seven Deep South states which had already declared their secession from the United States of America adopted the...

 during the American Civil War

American Civil War

The American Civil War was a civil war fought in the United States of America. In response to the election of Abraham Lincoln as President of the United States, 11 southern slave states declared their secession from the United States and formed the Confederate States of America ; the other 25...

. Confederate soldiers would use the yell during charges to intimidate the enemy and boost their own morale, although the yell had other uses. The exact sound of the yell is unknown and the subject of much speculation and debate. Likewise, the origin of the yell is uncertain. http://www.26nc.org/History/Rebel-Yell/rebel-yell.html

Units were nicknamed for their apparent ability to yell during battle.