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)
en-GB/ˈnjuːtɹəl ˈθɜːd/
en-US/ˈnutɹəl ˈθɝd/