The mirror (kagami) of Amaterasu-o-mi-kami which is one of the three imperial regalia (sanshu no shinki). The etymology is unknown but 'yata' is connected with 'eight'. Various sources record that the legendary emperor Sujin in the first century BC ordered it to be removed from the imperial palace. For up to a hundred years it was carried by the high priestess and temporarily installed for several years at a time in shrine after shrine before Amaterasu declared in the year 5BC, according to the Nihongi, that she was satisfied with the 'secluded and pleasant land' of Ise, the twenty-ninth and final resting place of the mirror. The replica of the mirror made for the imperial palace was damaged by fire on several occasions in the tenth and eleventh centuries and its fragments are now held in the kashiko-dokoro.
A Popular Dictionary of Shinto. Brian Bocking.