Amos's definition sounds about right to me. The impression I get of most (if not all) think-tanks is that when some politician or political group comes up with a highly controversial or generally half-assed idea, they go to a think-tank and say, "We want to do X, Y, and Z. Rationalize it for us so we can sell it to the public." A bit like an advertising agency, but with lots and lots of advanced degrees. Often they have "Foundation" somewhere in their name, which seems to give them credibility and an aura of non-partisanship.

Don Firth