A bet or wager.

“I had a gamble on the horses last weekend.”

A significant risk, undertaken with a potential gain.

A risky venture.

“The sailors had taken many gambles with the sea and always won.”

To take a risk, with the potential of a positive outcome.

To play risky games, especially casino games, for monetary gain.

To risk (something) for potential gain.

“He gambled his reputation on the outcome.”