Seinen
: 青年 【せいねん】
(n.) youth; young man.
Relatively uncommon in the west due to the emphasis on the male teen market, "seinen" is a demographic indicator for anime and manga aimed at a young adult male (college-aged) audience. As such, this kind of anime tends to be more sophisticated than shōnen anime. There are many of the same basic themes/subgenres as shōnen but they are more psychological, satirical, violent, sexual, etc. In other words they are intended for a more mature audience.
There is a stronger focus on plot and consequently less focus on action. Characters and their interactions are also often more developped than in shōnen anime. Because of this, seinen material is frequently misidentified as shōjo because it isn't very well known outside of Japan and because fans correctly recognize the character development, relationships and romance typical of shōjo. However, seinen anime usually deals with these subjects with a greater realism. Where shōjo will have an idealized love story, seinen will have more grays and uncertainties dealing with the practical give-and-take realities of a relationship.
Overall, seinen anime tends to be more strongly rooted in reality, with many incidental details added to heighten the sense of realism and even fantasy elements being subject to a strong "realistic" logic. Of course, it should be noted that those stylistic guidelines are a generalization of the genre and even an anime that has none of those characteristics can be classified as seinen if that was indeed the target audience. In fact, hentai (not including yaoi) is mostly targeted at the seinen demographic.
It should be noted that many of the works of anime most acclaimed for their depth and maturity (such as Patlabor, Maison Ikkoku and Ghost in the Shell) hail from the seinen genre.
See also: shōjo, shōnen, josei
Relatively uncommon in the west due to the emphasis on the male teen market, "seinen" is a demographic indicator for anime and manga aimed at a young adult male (college-aged) audience. As such, this kind of anime tends to be more sophisticated than shōnen anime. There are many of the same basic themes/subgenres as shōnen but they are more psychological, satirical, violent, sexual, etc. In other words they are intended for a more mature audience.
There is a stronger focus on plot and consequently less focus on action. Characters and their interactions are also often more developped than in shōnen anime. Because of this, seinen material is frequently misidentified as shōjo because it isn't very well known outside of Japan and because fans correctly recognize the character development, relationships and romance typical of shōjo. However, seinen anime usually deals with these subjects with a greater realism. Where shōjo will have an idealized love story, seinen will have more grays and uncertainties dealing with the practical give-and-take realities of a relationship.
Overall, seinen anime tends to be more strongly rooted in reality, with many incidental details added to heighten the sense of realism and even fantasy elements being subject to a strong "realistic" logic. Of course, it should be noted that those stylistic guidelines are a generalization of the genre and even an anime that has none of those characteristics can be classified as seinen if that was indeed the target audience. In fact, hentai (not including yaoi) is mostly targeted at the seinen demographic.
It should be noted that many of the works of anime most acclaimed for their depth and maturity (such as Patlabor, Maison Ikkoku and Ghost in the Shell) hail from the seinen genre.
See also: shōjo, shōnen, josei