Currently, a player needs to accrue an average of at least 3.1 plate appearances for each game his team plays in order to qualify for the batting title. An exception to this qualification rule is that, if a player falls short of 3.1 plate appearances per game -- but would still have the highest batting average if enough hitlessat-bats were added to reach the 3.1 average mark, the player still wins the batting championship.
The latest example of this exception being employed was in 1996, when Tony Gwynn had a .353 batting average, but only 498 plate appearances -- 4 short of the 502 necessary. Inasmuch as 4 additional hitless at-bats would have lowered his batting average to .349, but .349 was still better than anyone else in the league, Gwynn was named the National League batting champion. 
↑There has been contention that Nap Lajoie should be awarded the 1902 AL batting championship instead of Ed Delahanty because recent research by baseball historians has shown that Lajoie had a higher batting average. The 1902 AL batting championship was awarded to Ed Delahanty at the time, and MLB still credits Delahanty as the champion over Lajoie. The MLB Hall of Fame lists Lajoie's 1902 stats as being 129 H / 352 AB / .368 BA, but Baseball-reference.com lists them as 133 H / 352 AB / .378 BA, which would place Lajoie above Delahanty. However, Lajoie had only 371 plate appearances to Delahanty's 535 or 536 (depending on which source you use), so there is the issue of eligibility — Lajoie's 371 plate appearances falls well below the 424 that would be required under the current 3.1 plate appearances per team game standard of eligibility, and adding 53 hitless at bats would drop his average to .314. This discrepancy is somewhat significant, as Delahanty is regarded as the only player to win a batting title in both leagues (.410, 1899 Phillies), as of the end of the 2008 season.
↑Different sources credit both Cobb and Lajoie as the 1910 batting champion due to possible statistical errors. Cobb was awarded the title at the time. Research in the 1990s shows Lajoie probably should have been awarded the title. MLB continues to recognize Cobb as the title holder. Baseball Reference statistics for Nap Lajoie.