To gamble means to risk something valuable, especially money, on an outcome you can't control, hoping to win more than you started with. When someone gambles at a card game, they bet money on which cards will appear. When people gamble at a casino, they might play slot machines, roulette, or poker, knowing they could lose everything they wager.

Gambling works because the odds (the mathematical chances of winning) usually favor whoever runs the game, not the players. Over time, most gamblers lose more than they win. That's why casinos stay in business: they design their games so that players, on average, will lose. A person who gambles frequently or can't stop gambling even when losing money is called a gambler.

As a noun, a gamble is the risky bet or choice itself.

The word also means taking any risky chance. You might gamble on the weather by skipping your umbrella when clouds are gathering. A director gambles on an unknown actor for the lead role. A scientist gambles that years of research will lead to an important discovery. In these cases, you're not betting money, but you're still accepting risk with an uncertain outcome, hoping your choice will pay off.