An octave is a distance between two notes, it is an interval of 8 scale steps leading to the next note of the same name. According to frequency an octave is a doubling or halving of a frequency:
220, 440, 880, 1760 are octaves apart, 220 to 880 would be two octaves apart, 220 to 440 would be one octave apart. In music this measurement is made in scale tones as they would be in a 7 note major scale such as C major for example:
C D E F G A B C.
From any note in the chromatic scale to its next note of the same name is still called an octave apart, for example if starting on C :
C – C#/Db – D – D#/Eb – E – F – F#/Gb – G – G#/Ab – A – A#/Bb – B – C
This holds true for every note. Ranges of instruments or voices are measured in octaves, for instance the guitar has a range of approximately 3.5 octaves, a typical amateur vocalist would have a range of app. 1. 6 octaves.