prop bet
n.— «If placing a bet on either team or the over/under is just not enough to satiate your gambling jones, there are numerous other ways to find action—these wagers are called “prop bets.” Do you have enough confidence in the Cardinals to wager on them without taking the 7 points? Then play them straight-up and make more than 2-1 on your money. Does your gut tell you that Troy Polamalu will intercept a Kurt Warner pass? You can wager on that, too.» —“Super Bowl 43: Prop betting sheet” Sporting News Jan. 28, 2009. (source: Double-Tongued Dictionary)
Further reading
Old. Elderly. Senior. Why are we so uncomfortable when we talk about reaching a certain point in life? An 82-year-old seeks a more positive term to describe how she feels about her age. And: a linguist helps solve a famous kidnapping case, using the...
It may not be as rich a source of slang as baseball, but golf has contributed several terms to English, including stymie, “to get in the way of,” mulligan, a “do-over,” and par for the course, meaning “normal.”...
short for “proposition”…
http://en.wikipedia.org/wiki/Proposition_bet
More citations for “prop(osition) bet” here.