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....)
1. a small furrow or crease in the skin, especially of the face, as from aging or frowning.
2. a temporary slight ridge or furrow on a surface, due to contraction, folding, crushing, or the like.
The second meaning above describes uncrumpled paper perfectly:
I flattened the page out but it was still wrinkled.