What is a virtual package?

I'm trying to find the precise definition of what a virtual package is, i.e. when you type apt show linux-headers the output contains: linux-headers is not a real package (virtual) properties of linux-headers make it a virtual package?

Also, what are virtual packages good for?

I note that:
    apt-get -s install linux-headers
which says that linux-headers is a virtual package provided by a bunch of actual linux-headers-... and that I should install one of them explicitly.

I also noted that
dpkg -s linux-headers
does not include the term virtual at all. So are explicit: virtual packages only a apt concept and not a dpkg concept?