Octave
In music, an octave is when a melody goes up or down by eight notes. It’s like moving from one floor to another in a building. That’s an octave.
He moved the melody up by an octave. That’s like jumping to the top floor.
The singer went down an octave. That’s like falling to the basement.
They went up and down an octave. That’s like going from the ground floor to the top and back.