What do you call a web advert that obscures page content forcing you to look at it?

When you visit certain annoying websites, as soon as they load, they display a full-page advert that blocks the content. And you usually have to click "Skip ad" or similar to get rid of it.

Is there a name for that kind of ad?

Clarification: I don't mean those banner ads that expand when you roll over them, partially obscuring page content... I mean the big ads that appear before you see any content, when you first open a web page, almost like a splash screen. Sometimes they disappear after a few seconds, and sometimes you have to click to skip past them.

I believe the term you are looking for is "interstitial"; as defined on techtarget, it is

...a page that is inserted in the normal flow of editorial content structure on a Web site for the purpose of advertising or promotion. It can be more or less intrusive and the reaction of viewers usually depends on how welcome or entertaining the message is. An interstitial is usually designed to move automatically to the page the user requested after allowing enough time for the message to register or the ad(s) to be read.

(It further specifically identifies the "splash page" that automatically proceeds to the real site as a particular subset of interstitial advertising.)

(It is also defined on wiktionary, if you find that to be a more authoritative source....)

An ad (or any other sort of frame/window) that pops up and requires interaction is referred to as modal. It's a user interface design term. The use of modal windows is often discouraged, because they make the user feel handcuffed and frustrated.

It's called a shoshkele, sometimes also spelled soshkel/soshkele. It uses code to expand and fill the viewport.

I have seen them called pop-over ads, and sometimes over-lay ads.

I thought they were called rollover ads. But I could be wrong.

ETA- Splash pages possibly then.