Onionskin. With the exception of some of the antique German mibs, which are classified as having a certain kind of core (solid core, latticinio core, ribbon core, divided core, etc), most mibs are classified by what they look like on their surfaces. Like this one being an "onionskin" (you can't always see the interior of an onionskin -- the "skin" is super thin and applied over an ordinary clear glass -- and we wern't really meant to see into it).
A coreless swirl, basically, has the kind of outer decorative bands that the "cored" swirls do (latticinio, ribbon, etc.), but with nothing in the center. Hence "coreless." It's a term usually applied just to that kind of old German swirl.