Cesiumfranciolithicmyxialobidiumrixydixydoxidrexidroxhide (pronounced: sez-ee-um-frank-o-lith-ic-mix-ee-al-o-bid-ee-um-rix-ee-dix-ee-dox-ee-drex-ee-drox-ide) was an Alkali.
It was the antidote for the chameleonic virus that was destroying Red Dwarf (zogotheniumeliumoxiixiexiphulmifhidikaldrihide). Rimmer took a sample of the virus to a mirror universe to find a cure. The Professor identified it quickly and told Rimmer what it was. When Rimmer asked him to write it down, the Professor had to ask his secretary for an extremely long piece of paper. ("Only the Good...")
Behind the scenes
The name was scientifically nonsensical and meaningless, and was coined by the production team. It was given a long name as it was created for humourous effect.
In the deleted ending, it was said to be very famous (such as so that even Dave Lister could pronounce it) and another compound was named, with which it was not to be confused.
Doug Naylor and Ed Bye told Danny John-Jules that he did not have to learn how to say the name perfectly, but just make it up as he went. John-Jules refused to improvise and learned to say the long name flawlessly. This meant that the rest of the cast had to learn it just as well for the scene where they must all recognise the alkali. After all that hard work, the scene was cut from the finished episode, much to their dismay. Only John-Jules is heard to mention the full name.