1. A musical interval of eight notes. Every note has exactly double the number of vibrations per second of its corresponding note an octave lower and half the vibrations of its corresponding note an octave higher. 2. Term popularly used to describe thirteen equally-tuned half steps including the notes at each end (i.e., C to C, the twelve-toned equal temperament scale into which the octave is divided on the piano and in most pipe ranks). Sometimes called the chromatic scale. In original catalogs (e.g., Operators Piano Co. description of Coinola percussion) and in popular usage today a "two octave" set of xylophone bars would mean a 24-note chromatic set, for example. As noted, the 13-step terminology is also often used, especially in describing pipe ranks. 3. A 4′ pipe organ rank of flute or diapason tone. For example, Welte identified the 4′ flute rank in many orchestrions the "octave."