An Octave is an interval of eight (oct- means 8). It is used primarily in Western music. If two notes are one octave apart, the higher note has exactly a frequency twice that of the lower note. For example the central C has a frequency of about 260 Hz, the C an octave under is about 130 Hz and the C an octave higher is about 520 Hz. Thus eight tones above any pitch is another pitch that is interchangeable with it, sounds the same, even if higher, and has the same name. This means the two notes sound the same although they're eight tones, or twelve semitones, higher or lower.
Pocket Manual of Musical Terms, edited be Theodore Baker