opening poem; prefatory song
Refers to a poem or song placed at the beginning of a collection, anthology, or performance, serving as an introduction or opening piece.
The opening poem of this anthology is one that celebrates the arrival of spring.
The prefatory poem often reflects the compiler's sentiments.
序歌 is a more general term for an introductory poem, not necessarily at the very beginning of a volume.
Compound of 巻頭 (beginning of a book/scroll) and 歌 (song/poem). The exact historical derivation is uncertain, but the term is conventionally associated with classical Japanese poetry collections.