The title of the rulers of the Shire.
After the end of the North-kingdom and after Angmar had been overthrown the Shire-folk chose a Thain to take the place of the King. The first Shire-thain was Bucca of the Marish, from whom the Oldbucks claimed descent. He became Thain in 379 ShR (ThA 1979). The Thainship remained in the Oldbuck family until the Oldbucks moved to Buckland (ThA 2340), and the Thainship was given to the Tooks where it remained hereditary until the FourthAge.
Since emergencies rarely happened in the Shire and laws were voluntarily kept, the Thainship was more a nominal position than anything else.