What does 青年 Seinen mean?

Seinen means a young man, in the plain sense of a person in his twenties. Applied to manga it identifies the audience a magazine is sold to, which is a fact about publishing rather than a description of the story.

The characters

青 (sei, also read ao) is the colour blue, and by an old extension the state of being unripe or young. 年 (nen, also toshi) is a year. Together they give a young person of years, which Japanese narrows to a young man.

The same 青 does this work elsewhere. It appears in words for immaturity and inexperience, where the sense is closer to green than blue in the English idiom.

The everyday word

Ordinary and mildly formal. A newsreader would use it. So would a civil servant writing about employment among the young. It is not a word with any particular energy, and a Japanese adult hearing it would not think about comics at all.

What the manga term actually specifies

Japanese manga magazines are sold to defined readerships, and the four common labels name those readerships: boys, girls, adult men, adult women. Seinen is the third. A series is seinen because of the magazine it runs in, which is a commercial fact, decided by editors before a page is drawn.

This is why the label predicts so little. Seinen magazines carry office comedies, fishing stories, mahjong, quiet slice-of-life, historical drama and horror. What their contents share is a reader assumed to be an adult with adult patience, not a reader who wants blood.

Where the English meaning came from

English-speaking readers met the word through the titles that reached them first, and those skewed hard toward the violent and the bleak. If the seinen shelf is a short list of grim classics, the reasonable inference is that seinen means grim. The inference is wrong, but it was well earned by the sample.

The label then travelled without its context. Nobody outside Japan was buying the magazines, so the one piece of information the word actually carries, which magazine bought the serialisation, was invisible. What remained was a vibe, and the vibe hardened into a genre.

Why English had nowhere to put it

English publishing does not label books by intended reader in the same load-bearing way, outside children's and young adult categories. There is no English shelf called adult men, because the trade treats that as the default rather than a segment.

So the word arrived with no English slot to sit in. Genre was the nearest available shelf, and a demographic term got filed as a genre term. The same thing happened to shonen, and for the same reason.

A practical consequence

Two series with identical content can carry different labels, and often do, because the label records where an editor placed the series rather than what the series is. A story that moved magazines mid-run changes demographic without changing a line.