After using the iMac terminals, please make sure to log out by selecting “Log Out (username)” in the Apple menu. However, if you are unable to select the menu for some reason and need to force the machine to shut down, please press the power button located on the back of the iMac for more than five seconds. Please note that if you press the button only for a few seconds, the iMac will enter the Sleep Mode and will return to the previous screen when you move the mouse or by entering text.
The power button on the iMac