blitz is a sudden, overwhelming attack — originally an intensive aerial bombardment, and by extension any fast, concentrated campaign carried out with more force than the target can absorb.
The word is a 1940 English shortening of German Blitzkrieg, "lightning war," which entered English the year before to describe the German combined-arms doctrine of rapid, deep armored thrusts backed by air power. Blitz alone means lightning, traceable through Middle High German blicze to Old High German blecchazzen, "to flash," and to the Indo-European root bhel-, "to shine, flash, burn." Capitalized and definite, the Blitz names one specific campaign: the sustained German bombing of British cities from September 1940 into May 1941.
The civilian senses followed quickly and are now the commonest. A media blitz, an ad blitz, or a blitz of calls describes a campaign compressed into a short window for the sake of saturation, and the verb follows the noun. American football adopted the term in 1959 for a defensive play in which linebackers or defensive backs abandon pass coverage to rush the quarterback — a use that keeps the original logic intact, since the play trades safety for the chance to overwhelm before the opponent can set. Chess uses it for games played at drastically shortened time controls.
Across every sense the defining property is compression rather than magnitude. A blitz is characterized by the rate at which force arrives, which is what separates it from a siege, a campaign defined instead by its duration.