Collections of the Twenty-One Eras; twenty-one imperial waka anthologies
Refers to the twenty-one waka collections compiled by imperial command from the early Heian period to the early Muromachi period, starting with the Kokin Wakashū and ending with the Shinshoku Kokin Wakashū.
See also: 勅撰和歌集
The Nijūichidaishū is known as the collective term for the imperial waka anthologies.