In optics, an octave typically refers to a range of frequencies or wavelengths that spans a factor of 2. In other words, when dealing with optics, an octave encompasses a doubling of frequency or halving of wavelength.
For example, if you have light with a certain wavelength, the range from that wavelength to twice that wavelength would be considered one octave. Similarly, the range from that wavelength to half of that wavelength would also be one octave.
The concept of octave is commonly used in discussions about spectral bandwidth, frequency ranges, or wavelength ranges in the field of optics, especially when dealing with light sources or optical devices that operate over a broad range of frequencies.