The septimal whole tone is a musical interval not found in most western music. It corresponds to the difference between the 7th and 8th harmonics, and is wider than a standard whole tone, and narrower than a minor third. Harmonically, the septimal whole tone can function both as dissonance and consonance, depending on context; it tends to be less dissonant than a whole tone but more so than most of the thirds.
The septimal whole tone exists in an intermediate area between scale steps and familiar harmonic intervals like thirds: it is narrow enough that it can sometimes be perceived as a single step of a scale, but wide enough that it can be perceived as a larger leap or jump. All narrower intervals in 31-ET, from the whole tone down, tend to be perceived as a single step, whereas larger ones tend to be perceived as a jump or leap.
The perception of this interval as a single step or something greater depends greatly on context.
The septimal whole tone has the interesting property that it implies the top note, rather then the bottom note, as a root. Its harmonic function is thus very different from a normal whole tone, which tends to imply the bottom note as root.