There is no strict definition for the term rebel flag. But it generally refers to flags used by people who at one time or another, in history, took the opposite side as compared to those in control of a nation, organization etc. When discussing American flags, it is known that in the Civil War period, many Southern states did not stay with the Union and they wanted their own flags. Thus in the American history we see the development of the rebel flag quite active and prolific in that period. The name rebel flag actually comes from these Confederate or Rebel states which separated and wished for their own flag to be flown. Hence the name rebel flag.
The first American rebel flag ever was the Bonnie Blue. This rebel flag wasn’t ever adopted officially by the Confederacy but it was quite popular among the Americans at the time. The first rebel flag officially admitted by the Confederate states was actually the one called the Stars and Bars. This first rebel flag was used from March, 1862 until May, 1863. The problem with this first official rebel flag of the Confederacy was that it looked too much like the The Union Stars and Stripes. Therefore the Confederates decided that a change is needed. Another rebel flag design was proposed which was unique and, obviously different from the previous ones. Thus the most famos rebel flag came into being and it was known as the Confederate Battle Flag or the Southern Cross as it was used by the Confederate soldiers in battle.
The second Confederate flag with its design was adopted on the 1st of May of the year 1863, thus sending the Battle Flag as a canton on a white field. This second officially adopted rebel flag of the Confederacy was called the Stainless Banner. As it had so much white in its design, the Stainless Banner was usually easily mistaken for a surrender flag, so it had to be changed again. Therefore, a third official Confederate rebel flag appeared and got adopted in 1865, on the 4th of May. This was similar in design with a little exception, namely it had a wide red stripe on the fly end to differentiate it from a true flag. This third official rebel flag of the Confederacy was named the Last Confederate Flag and had a very short active duty life as the war was about to end.
Anyway the best known rebel flag of the Confederate states remains the Confederate Battle Flag, especially as it appeared a lot in a very popular TV show called the Dukes of Hazard, in which this rebel flag appears as painted on the top of the car that belonged to the Duke Boys. In addition, this rebel flag called the Confederate Battle Flag also appeared on numerous Southern state flags at different times in history. The only state which keeps even today the rebel flag as part of their state flag, is Mississippi.