six immortals of poetry
Refers to six notable Heian-period poets (Ariwara no Narihira, Ono no Komachi, Henjō, Kisen, Ōtomo no Kuronushi, and Fun'ya no Yasuhide) whose works were included in the Kokin Wakashū. A historical and literary term.
The Rokkasen are six poets selected in the preface of the Kokin Wakashū.
the thirty-six immortal poets
From 六 (roku, 'six') + 歌仙 (kasen, 'poetry immortal'). The term originates from the preface of the Kokin Wakashū (early 10th century).