Freeroll Definition
In poker, a freeroll has two distinct meanings. One applies to the play of a single hand, and the other describes an entire poker tournament. In playing a particular hand of poker, a freeroll is a situation that arises (usually when only two players remain) before the last card has been dealt, in which one player is guaranteed to at least split the pot with his opponent no. A tournament in which no buy-in or entry fee is required but a real money prize is awarded.
From Wikipedia
Negative freeroll is a term used in poker. It refers to a situation, usually occurring in no-limit or pot-limit when contemplating an all-in wager, where the player acting first checks in a situation where they would be forced to call an opponent's final bet (due to pot odds and the strength of their holding). If the opponent has a stronger hand, the opponent will most likely bet and the player will call and lose all their money regardless. However if the opponent has a weaker hand, betting may be the only way to get the opponent's money into the pot, as checking allows the opponent the opportunity to check in turn.
Negative freeroll is relatively common in low stakes amateur play, but in higher stakes play allowing a negative freeroll on the final round of betting is a serious blunder.
Hypothetical example
In a game of no-limit Texas hold'em, in heads up play on the river with an $8000 pot, both players have $2000 remaining behind. The board is 5♠ 6♠ 9♣ 9♠ Q♣. If the player acting first holds 6♦ 9♦ they must bet to avoid negative freeroll even though they are losing to an opponent holding 7♠ 8♠, a pair of Queens, or the 9♥ with a Queen. In this case the higher holdings will almost certainly bet and the player will be compelled to call due to pot odds and hand strength, while lesser holdings such as a lesser full house, a flush, or a straight that would have called the bet may simply check to see the hand down. Furthermore, with only $2000 remaining to bet in a $8000 pot, checking to induce a bluff is unlikely to succeed against competent opponents.
Freeroll Poker Tournaments Definition
Further reading
Freeroll Poker Definition
Harrington, Dan; Robertie, Bill (2006). Harrington on Hold'em: Expert Strategy for No-Limit Tournaments; Volume III: The Workbook. Two Plus Two Publishing, pp. 41-56. ISBN 1-880685-36-1.