User blog comment:Soulreaper1234/Urahara says Fusion Aizen says Subdue/@comment-1248963-20100508090042

In simple words I believe Aizen has no idea of what he's gotten himself into and that Urahara obviously knows more and has always known more about the Hogyoku than what Aizen initially thought, and this is what will become the clue point in Aizen's defeat. Though that's a theory, of course.

As for the base question, definitely Fusion. Polymerization if you want to (I should really stop referencing non-Bleach stuff). Subduing, dominating, sounds like he's really taking over the Hogyoku while I'm not really sure if he's doing that. If you ask me, and by the looks of his "transformation", I believe the Hogyoku is taking over Aizen's body, in a pretty parasite-like way.