Tankoubon. 単行本. It's not specific to hentai. It's a term that refers to bound collections of the comics which were previously printed in magazines or newspapers.
For example, the Japanese would call each of these a tankoubon:
Aniyome Ijiri, a hentai manga tankoubon
Genshiken, a manga tankoubon
Calvin & Hobbes, an American comic "tankoubon"
In America, mainstream comics like
X-Men or
Superman were never published in newspapers or magazines. They were always published, right from the get-go, as stand-alone comics. But in Japan, it's different. Comics share the same breathing space with other comics in enormous magazines, like Shonen Jump. Because of this, there developed a consumer demand for "pure collections" of their favorite comics, i.e. the fans wanted to
just have one large Naruto book and not five different magazines which each contained a chapter from Naruto, DragonBall, One Piece, and Rurouni Kenshin, and Yu-Gi-Oh. (anachronistic example, but you get the idea)
Same thing with hentai. A lot of people publish their works in hentai magazines alongside other authors. But the fans wanted, say, to have a book that was
just of Drill Murata's work. They didn't want to spend $50 for a year-long subscription to a comic magazine that they were only going to purchase for just one author's stories. They would prefer to spend $10 to get all of his stories in one book: the hentai manga tankoubon.