The pylorus is the muscle at the outlet of the stomach which controls the emptying of stomach contents into the small intestine.
Stenosis means a narrowing of a passage.
Pyloric stenosis is a narrowing of the pylorus. This occurs when the muscle around the pylorus thickens, making the passage very narrow. Therefore it is more difficult for feedings to empty out of the stomach. Three out of every 1000 babies born in the United States develop pyloric stenosis. It occurs 4 times more often in boys than in girls.