A Japanese term literally meaning beautiful teenage boy. The equivalent English concept is pretty boy. The term describes an aesthetic that can be found in disparate areas in Asia: a teenage boy, around 13 to 18 years old, whose beauty (and sexual appeal) transcends the boundary of gender or sexual orientation. Bishounen characters are fairly common in manga and anime; a heavy number of male characters show subtle signs of the bishounen style, such as slender eyes or a feminine face.
Bishounen are usually drawn delicately, with long limbs, silky or flowing hair, and slender eyes with long eyelashes that can sometimes extend beyond the face. It's best to tag images that focus on the character's beauty, so there tends to be a heavier emphasis on solo or duo images here. Group images are fine, so long as the majority of the males exemplify this trait.
As the name suggests, the term is best used when you can still tell that the character is a male. If there are remarkable feminine traits, consider adding androgynous. For characters where their feminine traits are at the point of making you believe that they're a female, use otoko no ko instead.
Note that with female-to-male genderswaps, this tag can apply if they possess the above-mentioned traits.
In English-speaking circles, bishounen is used somewhat more loosely than it is in Japan, effectively used for any male character with pseudo androgynous beauty, regardless of whether they are "shounen" or not (so variants covering those older than 18, ala biseinen, bichuunen, and bidanshi, find themselves incorporated, as well as those younger than 13).