Wiktionary
n. A plane located halfway between the suprasternal notch and the upper border of the symphysis pubis; this indicates the margin of the transpyloric plane, which in most cases cuts through the pylorus, the tips of the ninth costal cartilages and the lower border of the first lumbar vertebra.
Wikipedia
The Transpyloric plane, also known as Addison's Plane, is an upper transverse line, located halfway between the jugular notch and the upper border of the pubic symphysis. It is also said to lie roughly a hand's breadth beneath the xiphoid process of the human sternum. The plane in most cases cuts through the pylorus of the stomach, the tips of the ninth costal cartilages and the lower border of the first lumbar vertebra.