What is the "infield fly rule" designed to prevent?

Which commissioner led MLB from 1992 to 2015? What is "Tommy John surgery"? Which surgeon first performed Tommy John surgery in 1974? What is the "Mendoza Line" in baseball? What is a "balk"? What is a "sacrifice fly"? What is a "pinch hitter"? What is the name of the spring training league held in Florida?