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.
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.