Gekiga

Gekiga (劇画^?) is Japanese for "dramatic pictures." The term was coined by Yoshihiro Tatsumi and adopted by other more serious Japanese cartoonists who did not want their trade to be known as manga or "whimsical drawings".

It's akin to Americans who started using the term "graphic novel" as opposed to "comic book".

Not only was the storytelling in gekiga more serious but also the style was more realistic. 

Tatsumi began publishing "gekiga" in 1957. Gekiga was vastly different from most manga at the time, which were aimed at children. These "dramatic pictures" emerged not from the mainstream manga publications in Tokyo headed by Osamu Tezuka but from the lending libraries based out of Osaka. The lending library industry tolerated more experimental and offensive works to be published than the mainstream "Tezuka camp" during this time period.

By the late 1960s and early 1970s the children who grew up reading manga wanted something aimed at older audiences and gekiga provided for that niche.

In addition this particular generation came to be known as the "manga generation" and read manga as a form of rebellion (which was similar to the role rock and roll played for hippies in the United States).

Because of the growing popularity of these originally underground comics, even Osamu Tezuka began to display the influence of gekiga cartoonists in works such as Hi no Tori (Phoenix), produced in the early 1970s.

These kinds of works are now found in slightly more underground publications (usually seinen magazines).