Blitzkrieg (Lightning/Flash Warfare) is a word used to describe a combined arms surprise attack, using a rapid, overwhelming force concentration that may consist of armored and motorized or mechanized infantry formations, together with artillery, air assault, and close air support. The intent is to break through an opponent's lines of defense, dislocate the defenders, confuse the enemy by making it difficult to respond to the continuously changing front, and defeat them in a decisive Vernichtungsschlacht: a battle of annihilation.

During the interwar period, aircraft and tank technologies matured and were combined with the systematic application of the traditional German tactic of Bewegungskrieg (maneuver warfare), involving the deep penetrations and the bypassing of enemy strong points to encircle and destroy opposing forces in a Kesselschlacht (cauldron battle/battle of encirclement). During the invasion of Poland, Western journalists adopted the term blitzkrieg to describe that form of armored warfare. The term had appeared in 1935, in the German military periodical Deutsche Wehr ("German Defence"), in connection to quick or lightning warfare.

German maneuver operations were successful during the campaigns of 1939–1941, involving the invasions of Belgium, the Netherlands, and France and, by 1940, the term blitzkrieg was being extensively used in Western media. Blitzkrieg operations capitalised on surprise penetrations, such as that in the Ardennes forest, the Allies' general lack of preparedness, and their inability to match the pace of the German attack. During the Battle of France, the French made attempts to reform defensive lines along rivers but were frustrated when German forces arrived first and pressed on.

Despite being common in German and English-language journalism during World War II, the word Blitzkrieg was never used as an official military term by the Wehrmacht, except for propaganda, and it was never officially adopted as a concept or doctrine. According to David Reynolds, "Hitler himself called the term Blitzkrieg 'a completely idiotic word' (ein ganz blödsinniges Wort)". Some senior German officers, including Kurt Student, Franz Halder, and Johann Adolf von Kielmansegg, even disputed the idea that it was a military concept. Kielmansegg asserted that what many regarded as blitzkrieg was nothing more than "ad hoc solutions that simply popped out of the prevailing situation". Kurt Student described it as ideas that "naturally emerged from the existing circumstances" as a response to operational challenges.

In 2005, the historian Karl-Heinz Frieser summarized blitzkrieg as the result of German commanders using the latest technology in the most advantageous way, according to traditional military principles, and employing "the right units in the right place at the right time". Modern historians now understand blitzkrieg as the combination of traditional German military principles, methods and doctrines of the 19th century with the military technology of the interwar period. Modern historians use the term casually as a generic description for the style of maneuver warfare practised by Germany during the early part of World War II, rather than as an explanation. According to Frieser, in the context of the thinking of Heinz Guderian on mobile combined arms formations, blitzkrieg can be used as a synonym for modern maneuver warfare on the operational level.

The traditional meaning of "blitzkrieg" is that of German tactical and operational methodology during the first half of the Second World War that is often hailed as a new method of warfare. The word, meaning "lightning war" or "lightning attack" in its strategic sense describes a series of quick and decisive short battles to deliver a knockout blow to an enemy state before it can fully mobilize. Tactically, blitzkrieg is a coordinated military effort by tanks, motorized infantry, artillery and aircraft, to create an overwhelming local superiority in combat power, to defeat the opponent and break through its defences. Blitzkrieg as used by Germany had considerable psychological or "terror" elements, such as the Jericho Trompete, a noise-making siren on the Junkers Ju 87 dive bomber, to affect the morale of enemy forces. The devices were largely removed when the enemy became used to the noise after the Battle of France in 1940, and instead, bombs sometimes had whistles attached. It is also common for historians and writers to include psychological warfare by using fifth columnists to spread rumours and lies among the civilian population in the theatre of operations.

The origin of the term blitzkrieg is obscure. It was never used in the title of a military doctrine or handbook of the German Army or Air Force, and no "coherent doctrine" or "unifying concept of blitzkrieg" existed; German High Command mostly referred to the group of tactics as "Bewegungskrieg" (Maneuver Warfare). The term seems to have been rarely used in the German military press before 1939, and recent research at the German Militärgeschichtliches Forschungsamt, at Potsdam, found it in only two military articles from the 1930s. Both used the term to mean a swift strategic knockout, rather than a radically new military doctrine or approach to war.

The first article (1935) dealt primarily with supplies of food and materiel in wartime. The term blitzkrieg was used in reference to German efforts to win a quick victory in the First World War but was not associated with the use of armored, mechanized or air forces. It argued that Germany must develop self-sufficiency in food because it might again prove impossible to deal a swift knockout to its enemies, which would lead to a long war.