Definitions for "Extension tube"
Metal ring attached to a lens that extends the distance from film to lens so that it may focus at closer than normal distances.
a light-tight tunnel that extends the distance from your lens to the camera body, thereby increasing magnification--the greater the distance, the more magnification
a non-optical device which increases the distance between the lens and the film
On the DC11 Telescope vacuum cleaner this is also known as the "telescopic wand": it expands for use and can be compacted down for neat storage.