What is a "Bestseller"?

Whatever happened to the good old days when calling a book a "bestseller" really meant something?  Mike Banks asks that question on his blog and I think he makes some very good points.

What list is this a "bestseller" on?  The New York Times?  The Wall Street Journal?  Publisher's Weekly?  Barnes & Noble?  Amazon?  While it helps to quantify the claim (e.g., "more than a million copies sold"), I'm to the point now where "bestseller" feels like "whiter, brighter" on a box of laundry detergent: It's a phrase everybody uses and therefore has almost no significance.