Blowback refers to a particular system for implementing automatic re-loading for guns, the others being recoil-actuated and gas-actuated.
In a blowback system the expanding gases from the round being fired "blow back" the breech of the gun, which is left unlocked. This system is only really useful for smaller weapons with short barrels, where the breech can start moving as soon as the round is fired. With larger weapons the breech would move too far before the round left the barrel, and can spoil the aim. Thus blowback is typically found only on pistols and submachine guns.
The cyclic firing rate is usually very high, because the mass of the bolt determines how quickly the action operates. Users prefer light-weight weapons, and this causes the gun to cycle quickly. In a blowback firearm, the firing pin is often just a small projection on the bolt. When bolt slams home, the round fires. These guns usually stop with an open chamber. They are also rather unsafe, because a hard bump can cause the heavy bolt to spring the sear. The sear releases the bolt, and then the gun may fire.
Blowback is also a term used in espionage to describe unintended consequences. In context, it can also means retaliation as the result of actions you have undertaken, or revenge taken by those you have betrayed. It is believed to have been coined by the CIA. Given prior CIA support of the Islamic insurgency in Afghanistan and Osama Bin Laden, it could be argued that the September 11, 2001 Terrorist Attack is a monstrous case of blowback.
See also deniability.