Rock jock, that is not true by definition. "Constitutional" is not defined as "The Supreme Court says [whatever it is] is constitutional." I happen to believe that "Constitutional" has something to do with:
1) popular sentiment that a law is constitutional, or at least lack of strong sentiment that a law is unconstitutional
2) The Supreme Court deciding that a law or action is constitutional
3) A plain language reading of the Constitution. It's English, people.
4) Intent of the founders.
With criteria 3 and 4 being far and away the most important. Either 1 or 2 or both are optional, but they help the situation. It's important to remember that SCOTUS opinions are always changing. Racial equality proponents didn't let Plessy v. Ferguson stop them, even though the issue was, as far as the SCOTUS was concerned, decided in the previous century. What a disaster it would have been for social progress had those of discriminated-against races accepted Plessy v. Ferguson and not campaigned for equality.
A prevailing theory in legal circles seems to be some combination of 1 and 2, only. While I agree that, pragmatically speaking, those two criteria are the most important, I don't think they have much to do with the definition of "constitutional."
Criteria 1 and 2 are slightly more important when either the people or the SCOTUS or both view legislation or an executive action as unconstitutional. Given the structure of our Constitution, a random piece of legislation is more likely to be unconstitutional than it is to be constitutional; if the people or SCOTUS views something as unconstitutional, either or both are more likely to be correct. When either the people or the SCOTUS or both view something as constitutional, however, that doesn't mean as much. People are more easily swayed into believing society needs more regulation than into believing it needs less.
Again, I agree that SCOTUS decisions mean a lot for practical purposes... an individual isn't very likely to get anywhere in lower courts suing over 2A violations when the SCOTUS has ruled that there are none. I don't see how that relates to the meaning of the word "constitutional," though.