What is the term used to describe an augmented fourth or diminished fifth interval, consisting of three whole steps?

The term that describes an augmented fourth or diminished fifth interval, consisting of three whole steps, is known as a tritone. This interval is distinctive in Western music for its dissonant quality and has historical significance due to its tension-producing nature. The tritone spans six half steps, which can be perceived as three whole steps, thus aligning with the definition given in the question.

The augmented fourth can be viewed as expanding the perfect fourth by a half step, while the diminished fifth essentially reduces the perfect fifth by a half step. In both cases, the tritone serves as a bridge between these two intervals, showcasing its unique position in the scale. This makes the tritone an essential element in creating tension and resolution in various musical contexts, particularly within harmonic progressions and genres that emphasize dissonance.