oc·tave 
/ˈɑ:ktɪv/ noun
plural oc·taves /ˈɑ:ktɪv/ noun [count] music : the difference in sound between the first and eighth note on a musical scale
▪ He sang the song an octave lower. ▪ The two tones are (spaced) an octave apart.
▪ He sang the song an octave lower. ▪ The two tones are (spaced) an octave apart.
