What is a "pinch hitter"?

What is the "Mendoza Line" in baseball? What is the "infield fly rule" designed to prevent? What is a "balk"? What is a "sacrifice fly"? What is the name of the spring training league held in Florida? What is the name of the spring training league held in Arizona? What is the highest level of minor league baseball? In what year was the first MLB All-Star Game played?