From Unofficial Handbook of the Virtue Universe
A bishonen/bishounen character is an archetype drawn mainly from Japanese culture, particularly manga and anime. A bishounen is a male who possesses remarkable beauty of a somewhat feminine nature; slender builds, long hair and long-lashed feminine eyes are very typical bishounen traits. Some bishounen are indistinguishable from flat-chested females (jokingly called "traps" by western audiences), but the more classical bishounen design mixes feminine traits with masculine - for example, a girlish face/hair and slim figure coupled with a tall build, broad shoulders and angular body/face.
Though technically "shounen" refers to adolescent boys, adult characters (biseinen) are also typically called "bishounen" by Western fans. Thus, this category is appropriate for pretty-boys of any age.