A forecast is a prediction of what will happen. If the weather forecast calls for sunny skies, consider leaving your umbrella at home. (But, since forecasts aren’t guaranteed to be right, don’t blame us if you get rained on.)
While often used in the context of weather, forecast can also be used for other types of predictions such as those related to financial or political outcomes. Note that a forecast is typically a prediction made by experts. So, while a financial analyst might forecast fourth-quarter profits, your prediction about whether your friend will pay back the five dollars he owes you is more of a hunch than a forecast.