Kobetsu or oson is a term that refers to shizoku (clan, family), a branch family of a royal family or imperial family, especially a family which has seceded from Japanese Imperial Family.
皇別（こうべつ）もしくは王孫（おうそん）とは、王家や帝家、とりわけ日本の天皇家の一門の中で臣籍降下した分流・庶流の氏族を指す言葉である。