Well certainly over here in Blighty, the definition of a Multiple Unit is one which can work in multiple with others (as in the definition of my previous post) ....nothing very difficult about that.
Wiki itself has a definition here
http://en.wikipedia.org/wiki/Multipl..._train_control , but as always you need to remember that different countries may have significant variations.
Whilst the Railway itself is usually quite specific about definitions, there are always alternative names for things floating about in the 'lay' world. I certainly wouldn't lose any sleep over it.
Best wishes,
FP