I don't know if there is a precise definition, but to me, the fin world is divided into three categories: Split fins, which have splits; paddle fins, which have a single, flat blade, and "other", which includes Slingshots and Seawing Novas and Force Fins.
Paddle fins can vary enormously in stiffness. The Razors we bought for our OW class are almost as floppy as splits. The Dive Rite EXPs are pretty stiff. The Jets fall in the middle, and Turtles are a bit softer than Jets, as I understand it.
Paddle fins can vary enormously in stiffness. The Razors we bought for our OW class are almost as floppy as splits. The Dive Rite EXPs are pretty stiff. The Jets fall in the middle, and Turtles are a bit softer than Jets, as I understand it.