Trifecta, which made its print debut in 1974, is a combination of tri plus the fecta from perfecta. The trifecta is also called the triple: it names the bet in which a bettor must select the first three finishers of a race in the correct order of finish to win. The trifecta is a variation of the perfecta, which first appeared in print in 1967. In that bet, a bettor must select the first and second place finishers of a race in the correct order in order to win. Perfecta comes from the American Spanish quiniela perfecta (meaning "perfect quiniela"). Quiniela is the oldest of these terms, dating to 1905. The American Spanish quiniela is a game of chance resembling a lottery; in American English, the quiniela is a bet in which the bettor picks the first and second place finishers but need not designate the order of finish in order to win.

And if you'd like to know a little more about the Kentucky Derby, the Preakness, or the Belmont Stakes, or about horse racing in general, those links above take you to the appropriate entries in the Collegiate Encyclopedia.