main melody; lead
In music, the primary melodic line of a piece, as opposed to accompaniment or harmony.
The main melody of this piece is very easy to remember.
The violin carries the main melody.
central theme; main thread
Figurative extension: the core idea or recurring theme in a literary work, critique, or argument.
The central theme of this novel is 'loss and rebirth'.
Compound of 主 (しゅ, 'main') + 旋律 (せんりつ, 'melody'). 旋律 itself is a Sino-Japanese word composed of 旋 ('revolve, turn') and 律 ('rhythm, law').