Bishonen Jump Syndrome, also known as Shoujo Jump Syndrome and Odagiri effect, is a specific trend aimed at attracting a female Periphery Demographic through gratuitously bishounen or any attractive male character design.
The manga anthology magazine Weekly Shonen Jump (on which the US magazine Shonen Jump was based) has long been the best-selling manga magazine in Japan. Although it is targeted to boys, it has a large female Periphery Demographic. Sometime in the mid-1990s, Shonen Jump realized the sales potential of this demographic, and began to deliberately court female readers via Mr. Fanservice. Specifically, they took standard shounen all-male teams and simply made them into a Cast Full of Pretty Boys (an idea promptly picked up by shoujo as too good to waste on the guys). Traces of Ho Yay are often also added (if not, the fangirls will see it anyway).
Among the first series to show this effect were YuYu Hakusho and Rurouni Kenshin (albeit with the latter it was a deliberate choice by the mangaka); compare the art in these to Slam Dunk or Great Teacher Onizuka, which have old-school character designs. Even Hirohiko Araki mentioned deliberately going for a prettier look when he wrote Part 5 of JoJo's Bizarre Adventure, and the effect shows even in the Seinen-aimed installments. Katekyō Hitman Reborn!, D.Gray-Man, The Prince of Tennis and Kuroko no Basuke are probably the most blatant examples in recent times.
The fangirl-bait worked, sales went up, and the idea was promptly copied by other shonen magazines, to the extent that it is now more-or-less standard procedure. It is nonetheless often ridiculed by readers who prefer the older tough-looking art styles, who deride Shonen Jump as "Bishonen Jump" or "Shoujo Jump".