The terms are so imprecise that they tend to mean whatever the person speaking wants them to mean. But they are generally used to convey the same idea: a garment that is very soft; that has as little padding and canvas as the maker could pull off; and what is there is thin, light, and soft. Some do use the terms to refer to coats that are unlined or partially lined, but this is misleading. There are precise terms for that. The definition you suggest -- "nothing between a suit's lining and its outside" -- has some problems. First, even the most structured suit is going to have no padding between lining and shell (cloth) in the back, in the sleeves, and in most of the front below the waist. Second, even the most unstructured suit is going to have some canvas in the chest and lapels and at least a little wadding in the sleevehead. Third, a lot of what makes up the structure of a coat -- padding and canvas -- is not between the lining and the cloth but somewhere else.