A quarter is another way of saying one fourth, in this setting, one fourth of a year = three months. Quarterly means something that happens every 3 months. Edit
Outfielder Ty Cobb, whose career ended in 1928, has the highest batting average in Major League Baseball (MLB) history. He batted .366 over 24 seasons, mostly with the Detroit Tigers. In addition, he won a record 11 batting titles for leading the American League in BA over the course of an entire season.
Williams was an All-Star for seventeen seasons, an American League (AL) Most Valuable Player two seasons, an AL batting champion for six seasons, and a Triple Crown winner two seasons. He finished his playing career with a .344 batting average, 521 home runs, and a .482 on-base percentage, the highest of all time.