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. Like the Imperial Family, it must share the common agnate blood line. Kobetsu is a term associated with the classifications of three kinds of shizokus, namely Kobetsu (descendents of Emperors and Imperial Princes), Shinbetsu (descendents of Amatsukami [god of heaven] and Kunitsukami [god of the land]), and Shoban (descendents of immigrants from Korean Peninsula, Mainland China, and other areas) described in "Shinsen Shojiroku (Newly Compiled Register of Clan Names and Titles of Nobility), a genealogy of ancient cognate clans compiled by the Chotei (Imperial Court) in 815. Since the Edo period, the term 'oson' also has been used.
Kobetsu/oson existed from the ancient times when Emperor was called Okimi (great king), and were created by giving family names to branch Imperial families whose lineage to the Emperor became remote or to children whose mothers were mistresses or concubines of the Emperor in order to prevent financial troubles and succession races. They received a lot of respect as quasi-imperial families, but as the Fujiwara family established a political system which involved holding onto power as maternal relatives by marrying their daughters to members of the imperial family, most of the families fell into ruin during the second or third generations.
Some families which were derived from the Imperial family flourished as Toryo (leader) of samurai (warriors) in the provinces, and famous examples are Kanmu-Heishi (TAIRA clan) and Seiwa-Genji (Minamoto clan). In the 9th century, TAIRA no Masakado from Kanmu-Heishi, the 5th generation descendant of Emperor Kanmu, reigned over Kanto region as a new emperor for a short period of time. And it is said that the first buke (samurai family) who took control of the government in Japan was TAIRA no Kiyomori from Kanmu-Heishi. Seiwa-Genji established the bakufu (Japanese feudal government headed by a shogun) in Kamakura after MINAMOTO no Yoritomo defeated Taira clan's head family and became widely known as a leader of buke (military authority) through controlling the east Japan for three generations. Later the Ashikaga family, a branch family of the Minamoto clan established Muromachi bakufu, and partly because of the expansion of the power of bushi (warriors) and the decline of the authority and power of the Emperor, reigned over Japan as Seii Taishogun (literally, "great general who subdues the barbarians") and conducted foreign affairs, domestic administration, and military affairs over 15 generations for 240 years.
In the meantime, kobetsu shizoku (families derived from the Imperial family) were established by high-ranking court nobles adopting members of the Imperial family. Specifically the three families including the Konoe family after the adoption of Nobuhiro KONOE, the Ichijo family after Akiyoshi ICHIJO, and the Takatsukasa family after Sukehira TAKATSUKASA, their branch families that shares agnate blood lines and the families that adopted sons from these families are called Kobetsu Sekke (line of regents and advisers) today.
Being one of the branch families of the Imperial family gave the value to the family line, and not a few cases that persons pretended to be a member of Kobetsu/Oson are found in Japanese history. For example, Ieyasu TOKUGAWA made up a story that he belonged to Seiwa-Genji.
According to the definition, former Imperial Families are Kobetsu/Oson, but such appellation is seldom applied to these families.