Sales' prediction of how much revenue will close in a given period.
Forecasting is the practice of calling which deals will close this quarter and totaling them up. AEs forecast their own deals (often categorized: 'commit', 'best case', 'pipeline'), managers roll those up, and leadership gets a number. Good forecasting is ±5%. Bad forecasting is ±30%, and it destroys trust with the board.
Forecasting is how the company plans cash, hiring, and investor updates. If forecasts are unreliable, nothing else is. Good AEs and good managers forecast within a tight range and defend their calls with specifics. The rest guess.