An octave is an interval (8th) in which the two pitches played/heard are exactly the same. An octave is "the most fundamental interval in music" (www.fact-index.com) Notes that are an octave apart sound essentially the same to a human ear. "For this reason, notes an octave apart are given the same note name in the Western system of notation." Also, in the diatonic scale, the first and last notes are an octave apart. In sheet music, you might see "8va" notated. This means to play the selection an octave about what it is written.