1. A musical instrument with a fan-folded zigzagged air bag characteristic of the accordion family.
2. Figuratively, any object or shape resembling the unique appearance of accordions (for example, z-foldable doors, buses or trains with ruffled interconnecting segments, air-blowing devices, etc⋅). Specifically: An articulated bus comprising two or more rigid sections linked by a pivoting joint (articulation) enclosed by protective bellows inside and outside and a cover plate on the floor; banana bus; bendy bus; tandem bus; vestibule bus; wiggle wagon; stretch bus; accordion bus.
As soon as the first autobus had successfully departed, the concertina bus returned. Bending clumsily in the middle, it crawled towards the school building. (Source: J∙ Okram - The Mystery of the Rammed Key.)