It stands for Thin Plate Pure Lead. It's used to describe the plates which are at the heart of the technology. It's what makes Odyssey better than other batteries. The plates are actually a punched grid from continuous roll virgin lead so they are all exactly the same in every way size and dimension, weight, grid, unlike a cast plate which is never the same from pour to pour. Because of the consistent quality all batteries perform exactly the same which makes them easy to pair when matched sets are required.