In the description of verse forms, an octave is another name for what AWE refers to as an octet - any unit (or sub-unit) of eight lines. (These lines may be of any length, not necessarily the same; it is enough that they are perceived as a pattern which serves as a member of a poem, and may be repeated either within the poem or in all the examples of a particular type. All Shakespearian sonnets, for example, divide their fourteen lines into an octet and a sestet.)
There are certain other uses for the word octave. In Music, it is the most fundamental harmonic, being the higher note formed by halving the length of a string (doubling the frequency of the sound waves produced), or vice versa the lower note formed by doubling the length of a string (halving the frequency), Various other more technical uses, mostly related to the musical, are listed in OED.
- Some writers name the octave an octet. The terms appear indistinguishable.