I have seen the term 'Toast' used in the Windows 8 context. I have also seen them referred to as 'Toast Notifications'.
What exactly are 'Toasts' in Windows 8?
Windows 8 has a notification system similar to that implemented in mobile operation systems such as Android, IOS and Windows Phone.
A Windows 8 Toast is a notification triggered by an application or the operating system itself that is displayed to thew user by way of a small pop-up notification.
A toast is a small, informational window displayed by certain kinds of software, especially instant messaging clients such as AOL Instant Messenger, Windows Live Messenger, XFire and Trillian. Toasts notify users of various kinds of events, such as reception of new e-mail, a change in a peer's availability, or a change in network connectivity.
Toasts in Windows 8 comprise of a short text message and can optionally include a small image.
More technical info here.
They are "pop-ups". They pop-up like slices from the toaster. The name will mean little to the many peope who make toast under the grill.