It's not necessarily a dialog, but it's necessarily a modal window.
Think of it: it might not ask anything from the user (like a gallery viewer), but it doesn't allow you to interact with the rest of the page. Even if you click outside of it, you'll interact with the window (asking it to close), not the site behind.
Therefore, your system switch to a mode.
In old times, the computers have shown modal but not fullscreen windows only for dialogs. Hence the term "modal window" and "dialog" or "modal dialog" became synonyms (the only reasonable exception for a non-modal dialog is the Edit -> Find function in most apps, most times even preferences are modal)
A dialog is something which tells you something and waits for your answer. A form for example, is a dialog.
So it's not necessarily a dialog, but it's necessarily modal.
It's not a pop-up, as a popup is something which actually pops up system-wise. This doesn't necessarily "jump", it's not a system-wide feature, it's a page-wide feature.
I'm sure Cooper's About Face has more to tell you.