QC, the point with using a vague term like "syndrome" is, or at least should be, that
there is no other way to put a name on the characteristics that unite the sufferers. If there was a blood test, for instance, we could use "disorder" with a clear consciousness.
Down's, I believe, is a "syndrome" because of tradition, because it started that way, not because there isn't a better way to diagnose.
It also helps that some of the particular characteristics associated with Down's are rather easy to recognize.