Applications are never fully deleted. The “delete” button in the Context Pane of the App Builder moves that application to a trash can.
It can’t be run or modified when in the trash, but in can be recovered at any time.
To recover a deleted application, navigate to the group that the App was in. Click the “trash” icon on the bottom right of the screen. The icon only appears when there are items that have been deleted from that group.
In the menu that pops up, click “Restore” to recover the deleted app.