neutral interval
musical interval
neutral second
any musical interval wider than a minor second (semitone) but narrower than a major second (whole tone), e.g. those with frequency ratios of 12:11 (lesser undecimal neutral second) or 11:10 (greater undecimal neutral second)
neutral third
any interval wider than a minor third but narrower than a major third, e.g. with frequency ratios of 11:9 (undecimal neutral third), 16:13 (tridecimal neutral third) or with exactly 3½ semitones in equal temperament (equal-tempered neutral third)
neutral sixth
any musical interval wider than a minor sixth but narrower than a major sixth; e.g. 18:11 (undecimal neutral sixth), 13:8 (tridecimal neutral sixth), or the equal-tempered neutral sixth of exactly 8½ semitones