musical theme; motif
Refers to the central musical idea or motif of a piece, often used in classical or compositional contexts.
この交響曲の曲想はとても印象的だ。
The musical theme of this symphony is very striking.
Compound of 曲 (music, piece) and 想 (thought, idea). The exact historical derivation is uncertain, but the word is conventionally associated with the concept of a musical idea or motif.