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
en-US/ˈnutɹəl ˈsɪksθ/
en-GB/ˈnjuːtɹəl ˈsɪksθ/