Personally, I believe that an American comic is not manga, but I prefer to call things like NHS either Foreign-manga, Amerimanga, or (the catch-all) "Manga-inspired." That's what I would call an American comic drawn in the manga style.
Source: wiktionary
Every time the subject's come up, I've seen the complaint that "S/he's just copying the manga look, not the layout/storytelling/themes" bandied about over just about any Amerimanga artist you'd care to name. However, I'e ^([sic]) never really seen anyone describe exactly what they *meant* by that. Obviously, they mean more than (as Warren says) "huge eyes and speed-lines". So what else then?
Source: wiktionary
...and Devil's Due just keeps putting out more terrific, in-color Amerimanga (its new Udon line focuses on video game-related stories)...
Source: wiktionary
These titles, dubbed Amerimanga or original English language manga, draw their inspirations from Japanese and other Asian comics.
Source: wiktionary