For example, in this album cover:
Is the text "imposed" over the image? What is the right word for placing one layer of design on top of another?
This is overlay text. Text that covers the surface of the image is called overlay text. Because you can't see the parts of the image that are behind the white stroke of this font I would call it overlay :)
Text over an image.
Text on a background.
As Jenna answered, "overlay text" perhaps - not a term I'd use, but I'd have an idea about what someone meant if I heard it. "Overlay" to me, always infers opacity to a degree. Not the same thing in my mind as "text over an image".
I don't think there's a "special" term for it.