The output window in NetBeans contains the output of several processes. For example when we run Maven goals the output of Maven is displayed in the output window. After a while the output window can contain a lot of information. To get a clean slate we can remove all the output with a simple mouse click or shortcut.
In the output window we must right-click with the mouse and then select Clear from the pop-up window. Or we can use the shortcut Ctrl+L (in Windows) to clear the contents of the output window.