Senkai (仙界, hermit world) is a sealed secret world suited for hermits, which even Gensokyo did not know of before the events of Ten Desires. According to Toyosatomimi no Miko during the first part of a symposium between her, Kanako Yasaka and Byakuren Hijiri, it's a space that Miko had created, herself. A location where nobody else can bother someone is well-suited for hermits.