The notorious 88mm gun (commonly called the Eighty-Eight) was a German anti-aircraft gun. Widely used throughout the war, it was one of Germany's most recognized weapons.
'Flak' is a contraction of Flugzeugabwehrkanone, 'aircraft-defense cannon'. In English, 'flak' became a generic term for ground anti-aircraft fire. The Germans colloquially referred to the gun as the Acht-Acht ('Eight-Eight'). The 8.8 cm FlaK37 was the final development of the Eighty-Eight produced by Krupp. Weighing seven tonnes, it could fire 15-20 rounds per minute.
As well as its intended use in the anti-aircraft role, the 88 was also very successful as an improvised anti-tank gun, leading to the development of a tank gun based upon it: the 8.8 cm KwK 36.
In many parts of Germany, mobile FlaK units were employed with guns on railway mounts. These units moved from place to place in special FlaK trains, with their own living and kitchen accommodations.