The Mac OS Startup Screen is the initial image displayed on a Classic Macintosh following the "Happy Mac" icon that indicates bootable media with a proper System folder has been found. It is the first image displayed by the operating system to indicate it is loading onto the hardware.
From the original Macintosh System through System 7.5, this was a white rectangle with a black border displaying the words "Welcome to Macintosh". This was updated during the Macintosh System rebranding with Mac OS 7.5.3, such that Mac OS 7.5.3 and 7.5.5 quickly switch to a larger coloured square with the Mac OS logo. With Mac OS 7.6.1, this completely replaced the original Startup screen.
With specific extensions such as MACSBug installed, further text was also displayed in the dialog. In later versions of the operating system, further loading text was also displayed in this space, including "Extensions Off" if the shift key was held down during the boot process.
It was also possible in the Classic Mac OS to create a document containing a single PICT resource (ID 0? needs verification), name it StartupScreen, and place it in the System Folder. This would replace the white rectangle just prior to the loading of INIT resources/System Extensions.