An accordion is, as jQuery UI aptly puts it, a UI widget that “displays collapsible content panels for presenting information in a limited amount of space.”
It is a series of panels or content panes; only one may be visible at a time, and the header for each section is clickable/tappable, which causes that section to open (and the rest to close).
See it in actionEdit
Search the Web for “accordion demo” for more.
Opinions like this should be sourced, but often, a better alternative exists when an accordion might be used.
A disadvantage is that it separates options by inserting a block of content in what would otherwise be a continuous navigation.