The most general term for the basic subject matter of a piece of music is:

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The most general term for the basic subject matter of a piece of music is "theme." A theme is a melodic or rhythmic idea that is used as a basis for a composition or as a recurring element throughout a piece. While the terms "tune" and "melody" are related, they generally refer to a specific, recognizable sequence of notes, rather than a more general concept like a theme. A "phrase" refers to a smaller section of music that typically makes up part of a larger melody or musical idea.

The Most General Term for the Basic Subject Matter of a Piece