Wiktionary
n. (context music English) A blues form built on a twelve-measure chord pattern, most typically I,I,I,I,IV,IV,I,I,V7,IV,I,V7, where I, IV and V7 represent the tonic, subdominant and dominant seventh chords respectively.
Wikipedia
The 12-bar blues or blues changes is one of the most prominent chord progressions in popular music. The blues progression has a distinctive form in lyrics, phrase, chord structure, and duration. In its basic form, it is predominantly based on the I- IV- V chords of a key.
The blues can be played in any key. Mastery of the blues and rhythm changes are "critical elements for building a jazz repertoire".
Usage examples of "twelve-bar blues".
Catfish was a bluesman, and on the night before the night before Christmas, he was singing up a forlorn twelve-bar blues fog in the Head of the Slug saloon.
He launched into a twelve-bar Blues, playing the bass line with his thumb, squealing the high notes with the slide, rocking back and forth on the bar stool, the light of the neon Coors sign behind the bar playing colors in the reflection of sunglasses and his bald head.