Sorry that it is so difficult to describe this article.
It is a strong inflatable ball, about 55 cm diameter when inflated, that is designed to encourage comfortable upright seating and stability strengthening.
It looks like the one in the picture below, but without the frame, which appears to be a new addition since we got ours many years back.
It is in good condition and doesn't leak.
As always, reliable pickup is greatly appreciated.
Spherical Ergonometric ball chair - Spence