A super combinator is either a constant, or a Combinator which contains only super combinators as subexpressions.
This definition is bewildering until you realize that a Combinator can have non-Combinator internal subexpressions. A super combinator is "internally pure" (every internal lambda is a combinator) as well as externally.
Any Haskell program can be converted into super combinators using Lambda lifting.
See also Constant applicative form