What does 美少年 Bishounen mean?

Bishounen means a beautiful boy. The word is descriptive rather than technical, and a Japanese speaker using it is commenting on a face, not invoking a category.

The characters

美 (bi) is beauty, the same character in words for fine art and for beautiful scenery. 少年 (shounen) is a boy, built from 少, few, and 年, year: a person of few years.

That second half is the same compound that names the shonen publishing demographic, which is a useful accident. It shows how ordinary the parts are: nothing in the word points at comics, romance, or a style of drawing.

The pattern is productive

Swap the second element and the counterpart appears, formed identically for a girl. Swap the first and other compounds emerge in the same shape. Japanese builds descriptions this way constantly, and none of the results are terms of art.

Which is the whole correction. English received a common adjective-plus-noun compound and filed it as a specialist term, the way it did with the demographic labels.

What English does with it instead

Anglophone fandom uses it to name a character type: slender, delicate, long-lashed, styled to be attractive in a way that declines the usual signals of masculinity. That type genuinely exists in manga and has a long history in shoujo-adjacent art, so the usage points at something real.

It is still a different thing from what the Japanese word says. One is a description available for any beautiful boy, including a perfectly conventional-looking one. The other is a specific aesthetic with a lineage.

Why the narrowing happened

English needed a name for the aesthetic and did not have one. The available options were unhelpful: pretty boy is faintly mocking, and androgynous is clinical and often inaccurate, since many of these characters are not ambiguous so much as beautiful under a different set of conventions.

So a word arrived attached to exactly the characters English wanted to name, and it got recruited for the job. That is a reasonable outcome for English and a poor guide to Japanese.

The register, which is the part usually missing

It reads as slightly formal, a shade literary, and sincere. Applied to a real person it is a straightforward compliment, not a wink and not a genre reference, and it carries no implication about who the person is attracted to. English usage sometimes smuggles that implication in, and the Japanese word does not supply it.