intervals
Difference in pitch between two successive notes (melodic interval) or simultaneous notes (harmonic interval).
octave 
Interval of eight tones (e.g., from C to C).
sixth 
Interval of six tones (e.g., from C to A).
fourth 
Interval of four tones (e.g., from C to F).
second 
Interval of two tones (e.g., from C to D).
seventh 
Interval of seven tones (e.g., from C to B).
fifth 
Interval of five tones (e.g., from C to G).
third 
Interval of three tones (e.g., from C to E).
unison 
Notes of the same pitch, or one or more octaves apart, produced simultaneously by several voices or instruments.