Structure is not a specifically musical term. The term is used by musicians when they refer to the overall shape, design, and interrelationships within some larger musical parameter. Thus it is common to speak of a “melodic structure” when describing the contour, intervals, length, and other characteristics of a melody, or of a “harmonic structure” when describing the musical “architecture” provided by use of particular progressions of chords.
Courts use this term with in an even vaguer fashion, as found, for example, in the Ninth Circuit’s Newton v. Diamond opinion in which it set forth the criteria for the “fragmented literal similarity” test: “Fragmented literal similarity exists where the defendant copies a portion of the plaintiff's work exactly or nearly exactly, without appropriating the work's overall essence or structure.” Courts signal their uncertainty in using the term as applied to musical works qualifying it with adjectives like “overall” and “general”(e.g. Glover v. Austin, Newton v. Diamond). In such cases, it seems that courts use the term as being roughly synonymous with form at the highest level.