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)
en-GB/ˈnjuːtɹəl sɪˈkɒnd/
en-US/ˈnutɹəl ˈsɛkənd/