Octave is a Latin term for eight. In music, an octave is the interval between one musical pitch and another with double its frequency, or the distance between notes of the same letter name, eight notes higher or lower. Ie, in the key of C, the notes range from C to C, where the second C sounds twice as high in pitch as the first C and is an octave above in pitch. Twelve semitones equal an octave, so do the first and the eighth (hence octave) note in a major or minor scale.
Understanding the concept of octaves helps master musical scale theory. Most musical scales are written so that they begin and end on notes that are an octave apart. In today's standard concert A frequency of 440 Hz, an octave up is 880Hz and an octave down is 220Hz. In Western music notation, notes separated by an octave (or multiple octaves) have the same letter name and are of the same pitch class. The octave above or below an indicated note is sometimes abbreviated 8a or 8va (Italian: all'ottava), 8va bassa (Italian: all'ottava bassa, sometimes also 8vb), or simply 8 for the octave in the direction indicated by placing this mark above or below the staff.