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In gambling, the term "proposition bet" (prop bet, prop, exotic, novelty, or a side bet in the first context below) has two definitions.
Primary definition 
In most contexts, "proposition bet" denotes a bet made regarding the occurrence or non-occurrence during a game (usually a gambling game) of an event not directly affecting the game's final outcome. Examples include:
- Betting on which specific sub-outcomes occur during a game in which multiple sub-outcomes normally have the same value, for example:
- Betting on the amount of balls/strikes thrown by an American baseball pitcher, which is where the proposition bet was said to have originated. 
- Betting on which team scores first in a match
- Betting on the specific face value of one or more individual dice in craps
- Betting on which cards will make up the flop in a hand of poker
- Betting on which cards are dealt in blackjack: Over 35 different blackjack side bets are widely recognized in casinos. On some bets, such as the "Lucky Ladies" side bet that wins only if a player is dealt the queen of hearts and the dealer receives blackjack, the odds offered may be as high as 1000-to-1. These bets are rarely favored by experienced gamblers as the house edge on them typically exceeds the edge in a normal game. , while blackjack itself has a house edge of less than 1% if the player follows basic strategy.
- Betting on specific aspects of the pageantry surrounding a sporting event, such as a) the duration of the word "brave" in the pregame performance of the U.S. national anthem or b) the color of the Gatorade or other sports drink dumped on the winning team's coach in the traditional practical joke and victory ritual known as "icing"
Secondary definition 
Less commonly, "proposition bet" (in this context also "one-of-a-kind bet") denotes a dare for money; phrased in terms of the above definition, the bet is on whether the "dared" person will make a given event happen. If the "dared" person accepts the bet, he will collect if he succeeds or pay if he fails.
For an example detailed in the book The Professor, the Banker, and the Suicide King, David Grey once bet Howard Lederer, a professional poker player who became vegetarian following gastric bypass surgery, $10,000 that Lederer would not eat a cheeseburger. Lederer accepted the wager, ate the cheeseburger, and won the bet.