Improbable means unlikely to happen or be true. When something is improbable, the odds are against it, though it's not completely impossible.

If you've never played chess before and sit down to face a grandmaster, an improbable outcome would be winning the match. It's not technically impossible, but the chances are extremely slim. When a weather forecast says there's only a 5% chance of rain, rain is improbable that day.

The word often describes surprising events that happened despite long odds. An improbable comeback occurs when a team trailing badly in the fourth quarter somehow manages to win. An improbable friendship might form between two people who seem to have nothing in common. Scientists might make an improbable discovery when an experiment produces completely unexpected results.

Notice the difference between improbable and impossible: improbable things can happen, they're just not likely to. It's improbable that you'll flip a coin and get heads ten times in a row, but it's not impossible. However, it's impossible to flip a coin and have it land on both sides at once.

The related word improbability describes the quality of being improbable: “Despite the improbability of success, the inventor kept working on her flying machine.”