What is the meaning of Accordion?

A box-shaped musical instrument with means of keys and buttons, whose tones are generated by play of the wind from a squeezed bellows upon free metallic reeds.

A vertical list of items that can be individually expanded and collapsed to reveal their contents.

(Can we verify this sense?) A set of items (concepts, links, or otherwise) that can be packed and unpacked cognitively, or their representation as a set of virtual objects.