Octave

Etymology

From Latin "octava (dies)," the eighth day, from "octo" (eight). It is the eighth note of the diatonic scale, counting inclusively from the starting note — do up to the next do.

noun · /OK-tiv/
The interval between one note and another with double its frequency, which the ear hears as the same note higher or lower.
What it names
The strange fact that a note and its double sound like the "same" pitch — the frame that makes scales, singing ranges, and "the same tune, up an octave" possible.
Example
She couldn’t reach the high note, so she dropped it an octave and it still landed.
Related idea
An octave is one specific interval; a scale is the ladder of notes that fills the space within it. The scale supplies the steps; the octave is the full span from a note to its double.