Syndrome Without A Name (SWAN) is the name given to any pathological set of symptoms that does not represent any of the known diseases. In other terms, SWAN simply means "an unknown disease".
It has been estimated that up to 40% of all disabled children in the UK, or 6,000 per year, do not have a diagnosis and their condition is thus termed "SWAN".
References
Syndrome Without A Name Wikipedia(Text) CC BY-SA