What is the meaning of Accordion?

A box-shaped musical instrument played by compressing or expanding its bellows while pressing its buttons or its keys, causing pallets to open, which allow air to flow across strips of brass or steel (reeds).

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

Something, or set of things (concepts, etc), which can be expanded (or extended) and collapsed, like the musical instrument's pleated, folding bellows.