Literally "young adult." Used to describe manga aimed at males over the age of 16. Often covers the same subject matter as shônen manga, but with college-age or adult male protagonists and more overt sexual elements. The majority of the manga series currently reviewed on MangaManiacs.org are seinen series, possibly because it seems that many (if not most) of the manga series being translated by American companies are seinen series.
A genre aimed at older men.
Young man. Used as a label for stories aimed at men in their teens or early twenties (generally sports, war, or adventure stories).
This is anime aimed at a male, college aged, audience. It tends to have a firm basis in reality even if the content is rich in fantasy.
Seinen means "young Man." Manga and anime that specifically targets young adult males around the ages of 18 to 25 are seinen titles. The stories in seinen works appeal to university students and those in the working world. Typically the story lines deal with the issues of adulthood. The tough guy secret agent Golgo 13 is a good example of a seinen title.
Sometimes it is classified as shÅjo or shÅnen, but it has distinct features, usually classified by a wider variety of art styles and more variation in subject matter, ranging from the avant-garde to the pornographic. The female equivalent to seinen manga is josei manga. The genre is comparable to the English terms and genre "Adult".