A major/non-perfect interval lowered by a half step is known as what?

The correct answer is that a major/non-perfect interval lowered by a half step is known as a diminished interval.

To understand this, it's important to recognize the definitions of interval types. A major interval is one that is one whole step larger than a minor interval. When you lower a major interval by a half step, you produce a diminished interval. For example, if you take a major third (which consists of four half steps) and lower it by a half step, it becomes a minor third (three half steps), but if you lower it again, it turns into a diminished third, which consists of two half steps.

This principle holds true for other intervals as well. A perfect interval, when diminished, also results in a diminished interval. However, the focus here is specifically on the transformation of a major interval.

The other options are incorrect as they refer to different concepts: a minor interval is one half step smaller than a major interval, not that it can be turned into by lowering; an augmented interval is one half step larger than a perfect interval, and a perfect interval remains perfect unless diminished or augmented.