AgX: It is the common term for a sheet film insert for a developing tray that keeps sheets separated. It allows you to tray develop multiple sheets without manual handling (shuffling) - although obviously not as many sheets at the same time. It is also very useful for applying intermittent and/or reduced agitation techniques to sheet film development.

Although I'm not certain of this, I believe John Sexton may have coined the term. At least that was they guy I first heard use the word.