Microsoft mentions the toast as a visual element in the package manifest for Metro-style apps with the attribute
What does this mean?
A Toast is a non modal, unobtrusive window element used to display brief, auto-expiring windows of information to a user. Android OS makes relatively heavy use of them.
Here's an example of a Google Chrome toast notification on Mac OS X:
A list of descriptions of Toast windows on multiple platforms:
Luke Wroblewski in this blog post describes a use of this real time notification:
.. and added a site-wide system of real time notifications that tell you when someone you follow on Bagcheck creates, likes or comments on something. You can see one of these notifications in action in the animated image below. A little "toast" window appears in the upper right hand corner of your screen when someone cooks up a new update.
Also worth noting from the Android Developer Guide:
but it does not take focus (or pause the current activity), so it cannot accept user input
I learned the term from working with Android as well. We use them all the time to provide brief notifications "Item Saved to Favorites" is a common one. I think the biggest difference from a pop-up is that they are "auto-expiring." So anything with an OK/Cancel option would not be considered a toast. As for the origin of the term it could either be a reference to it popping up like toasted bread or like a dinner party toast "Something happened..Cheers!"
As this is an outdated thread I think it is important to add 'recent' information about "Toasts" notifications
From Google Android:
The Snackbar class supersedes Toast. While Toast is currently still supported, Snackbar is now the preferred way to display brief, transient messages to the user.
Alternatives to using toasts If your app is in the foreground, consider using a snackbar instead of using a toast. Snackbars include user-actionable options, which can provide a better app experience.
If your app is in the background, and you want users to take some action, use a notification instead.
From Material UI:
Generally, snackbars are the preferred mechanism for displaying feedback messages to users, as they can be displayed in the context of the UI where the action occurred. Reserve Toast for cases where this cannot be done.
It means the application can provide alerts or notifications. From Metro Style Apps - VisualElements
Here is more information: Creating and managing tiles, toast, and Windows push notifications and Quickstart: Creating a toast notification
Editor's info: Above links are from 2011 (or beginning of 2012). Microsoft has changed a lot since then.