A unofficial award in baseball. Usually defined as the batter with the best batting average, and most RBIs and Home runs. Yaz was the last to win it.
The winner of the Belmont Stakes, Kentucky Derby and Preakness Stakes.
Any individual who wins three different awards in an area.
The crown worn by the pope during the Reformation.

In the United States, a horse who wins the Kentucky Derby, the Preakness Stakes and the Belmont Stakes. Since most horses are born in the spring, they will be barely three years old and have less than one year of race experience (usually just a handful of starts) before the Derby. For a young, inexperienced horse, running over a mile at top speed can be very difficult, and to do it three times in five weeks has proven almost impossible. In addition, the Triple Crown races are only open to three year olds so each horse has just one chance.

In recent years, VISA has sponsored a Triple Crown Challenge, offering five million dollars to the owner of a horse that wins the Triple Crown. So far, no one has been able to claim this prize. There have been eleven Triple Crown champions in history, but none since 1978. More commonly, a horse wins two out of three races. The various race distances contribute: good sprinters may win the shorter contests but tire in the 1 1/2 mile Belmont. Conversely, horses with plenty of endurance might not have the quick speed needed to win the 1 3/16 mile Preakness. In this extremely competitive industry, it takes an all-around champion horse to add a name to the list of Triple Crown winners.

Winners of the Triple Crown:

• 1919 Sir Barton
• 1930 Gallant Fox
• 1935 Omaha
• 1937 War Admiral
• 1941 Whirlaway
• 1943 Count Fleet
• 1946 Assault
• 1948 Citation
• 1973 Secretariat
• 1977 Seattle Slew
• 1978 Affirmed

The Triple Crown is also one of the laurels on offer in the Six Nations Rugby Championship. It is contested only by the home unions, i.e. England, Wales, Scotland and Ireland (the teams which originally made up the Four Nations championship, before the addition of France and later Italy). If one of these teams defeats each of the other three, they are awarded the Triple Crown; for example, if Ireland were to win their Six Nations matches against England, Scotland and Wales.

The Triple Crown is one of the many sub-challenges within the Six Nations. Others include the Calcutta Cup which is contested between England and Scotland.

Focusing on baseball, below's a list of batting Triple Crown winners (leading their league in batting average, home runs, and runs batted in).

Year  League Player, Team                  AVG  HR   RBI
1878    NL   Paul Hines, Providence       .358   4    50
1894    NL   Hugh Duffy, Boston (Braves)  .438  18   145
1901    AL   Nap Lajoie, Phila. (A's)     .422  14   125
1909    AL   Ty Cobb, Detroit             .377   9   115
1912    NL   Heinie Zimmerman, Chicago    .372  14   103
1922    NL   Rogers Hornsby, St. Louis    .401  42   152
1925    NL   Rogers Hornsby, St. Louis    .403  39   143
1933    NL   Chuck Klein, Phila.(Phillies).368  28   120
1933    AL   Jimmie Foxx, Phila. (A's)    .356  48   163
1934    AL   Lou Gehrig, New York         .363  49   165
1937    NL   Joe Medwick, St. Louis       .374  31*  154
1942    AL   Ted Williams, Boston         .356  36   137
1947    AL   Ted Williams, Boston         .343  32   114
1956    AL   Mickey Mantle, New York      .353  52   130
1966    AL   Frank Robinson, Baltimore    .316  49   122
1967    AL   Carl Yastrzemski, Boston     .326  44*  121
*= tied for league lead

Note that the batting Triple Crown has not been won since 1967.

The term Triple Crown is less frequently used in baseball for pitchers who top the league in ERA, wins, and strikeouts in the same season.
The feat has been accomplished 32 times (33 if you count the 19th century American Association, which was a major league at the time).
Most recently, Pedro Martinez won the pitching Triple Crown in 1999.

Thanks to AllergicToEverything for correcting my spelling on Jimmie Foxx (not "Jimmy")