What defines a diminished interval?

A diminished interval is specifically defined as a perfect interval that is lowered by a half step. This means that if you take a perfect fifth, which consists of seven semitones, and lower it by one semitone, it becomes a diminished fifth, consisting of six semitones.

In the context of intervals, a perfect interval is one that traditionally includes perfect fourths, perfect fifths, and perfect octaves. By lowering a perfect interval by a half step, the relationship of the notes changes, resulting in a diminished interval. This alteration affects the harmonic and melodic functions of the interval within musical contexts.

Other choices focus on different aspects of interval modification that do not align with the definition of a diminished interval. For example, lowering an interval's root note by a whole step does not accurately define the relationship specific to diminished intervals, as it refers to a general alteration rather than a direct reduction of a perfect interval by a half step. Therefore, the most precise way to describe a diminished interval is through the lowering of a perfect interval by a half step.