What is the official term of a plane flying under another plane to avoid radar? I was watching the news and, just now, learned that the Russians used a "trick" by flying a fighter plane under a cargo plane to Syria and successfully avoided radar detection for the fighter plane. 

I wanted to learn more about this method (a Wiki article, for instance) but failed to find anything. So do the experts here know what this term is called, if there even is one?
My copy of "EW 101: A First Course in Electronic Warfare" calls it "formation jamming".



Mind you, an operator of a radar with non-cooperative threat recognition capability will at least suspect something is wrong (although not from all aspects and ranges).