What is Seinen? A type of anime or manga directed at a particular demographic! More info here.
Seinen (青年) is a Japanese term used to categorize anime or manga commonly aimed at men 15 and up who can read a large variety of Kanji (for Manga). The word Seinen actually means young man or youth.
This type of anime and manga may feature a wide variety of story lines and genres, but what makes it different from the other types is the stronger sexual tone and adult language (profanity). Of course, there are limits to the adult content, it doesn’t get as bad as Doujinshi can. Notable examples of Seinen anime and manga include: Akira, Battle Royale, Berserk, and Ghost in the Shell.
This is just one of five different types of Anime and Manga directed at certain demographics. For more demographic groups, see my articles on Kodomomuke, Shounen, Shoujo, and Josei. And don’t forget, you can find more information on subculture terms just like this in my growing Otaku Encyclopedia.