Chances are you've heard of the terms "comic books," "mangas," and "graphic novels." Will Eisner, responsible for popularizing the term "graphic novels," made a goal for comics to be seen as literature with critically acclaimed stories like "A Contact With God." Since then, writers like Art Spiegelman who wrote the classic "Maus," adopted the term to refer allegedly to a more mature form of comics.
Manga is important to Japanese art culture—many will be familiar with the original comics or anime adaptations of "Dragon Ball," "Sailor Moon," and "One Piece." It doesn't change the fact that calling these comics is still correct.
A comic is a story often told with visuals in multiple panels. Comic strips, comic books, graphic novels, and Mangas all have this, making them comics.
Take Charles Schulz's "Peanuts," one of the world's most popular four panels consisting of Charlie Brown and his friends' misadventures. Through decades of change within the characters and numerous re-releases of books, including the fantastic Fantagraphics Books, one could argue that these are comic books, graphic novels, or even manga if you read a Japanese translation.
It does not change the fact that "Peanuts" tells its stories through visuals, accompanied by speech bubbles and panels.
As for Manga—remember that the English translation of Manga (漫画 in Japanese using Kanji characters) is "comics." That means if someone reads Superman, an American creation in a Japanese translation, especially in Japan, they are reading a Manga. That doesn't mean they are wrong for calling Superman a comic, certainly not more so than reading a Dragon Ball comic in English oversea and calling it a comic book. There are no art styles exclusive to Japan, America or any other nations.
It's like arguing that anime is a different media from animation, when the only difference is that anime animation made in Japan.
Further confusing is the false assumption that comic books and graphic novels are somehow different. Doing so implies that comic books is inherently juvenile while graphic novels is the sophisticated medium made for adults. Despite this, nothing stops a comic book from being mature or a graphic novel from being childish.
It certainly hasn't stopped the Japanese comic industry from categorizing their comics into five different sections, kodomo (children), shonen (teenage males), shojo (teenage females), seinen (adult male), and josei (adult women), and still calling all of it comics.
Had Eisner not distributed "A Contract With God" through bookstores instead of comic shops that were popping up (granted not without justifications), there would be no doubt that graphic novels are comics.
There's no shame in calling a comic a comic. Either way, it's the medium that follows the most beloved gang of children being mean to each other like in "Peanuts," or follows the journey of a middle-aged man who lost his parents at a young age and now fights crime against Gotham's worst enemies like in Batman. It's the search for seven mystical Dragon Balls to grant a wish like in "Dragon Ball," or provide thoughtful commentary on the decline of superheroes in "The Watchmen."
So relax and enjoy them for what they are.