What is Octave?

Legal Definition
In music, an octave (Latin: octavus: eighth) or perfect octave is the interval between one musical pitch and another with half or double its frequency. It is defined by ANSI as the unit of frequency level when the base of the logarithm is two. The octave relationship is a natural phenomenon that has been referred to as the "basic miracle of music", the use of which is "common in most musical systems".

The most important musical scales are typically written using eight notes, and the interval between the first and last notes is an octave. For example, the C major scale is typically written C D E F G A B C, the initial and final Cs being an octave apart. Two notes separated by an octave have the same letter name and are of the same pitch class.

Three commonly cited examples of melodies featuring the perfect octave as their opening interval are "Singin' in the Rain", "Somewhere Over the Rainbow", and "Stranger on the Shore".

The interval between the first and second harmonics of the harmonic series is an octave.

The octave has occasionally been referred to as a diapason.

To emphasize that it is one of the perfect intervals (including unison, perfect fourth, and perfect fifth), the octave is designated P8. The octave above or below an indicated note is sometimes abbreviated 8a or 8va (= Italian all'ottava), 8va bassa (= Italian all'ottava bassa, sometimes also 8vb), or simply 8 for the octave in the direction indicated by placing this mark above or below the staff.
-- Wikipedia
Legal Definition
In old English law. The eighth day inclusive after a feast; one of the return days of writs. 3 Bl. Comm. 278.
-- Black's Law Dictionary
Legal Definition
The eighth day after a feast day.
-- Ballentine's Law Dictionary