An octave is a frequency ratio of 2:1. An octave band consists of all of the frequencies within an octave. There is one octave between 100 Hz and 200 Hz, and between 1,000 Hz and 2,000 Hz, for example. Octaves are perceived as equal pitch intervals, even though the true bandwidth in Hertz varies with the frequency level of the octave. To our ears, two frequencies an octave apart (whether it might be 100 to 200 Hz or 1,000 to 2,000 Hz) sound like the same note.
It is interesting that our ears obey a precisely logarithmic law when assigning subjective pitches to frequencies. Even though an octave is strictly speaking a subjective judgment, it is so closely equal to frequency doubling (no matter where one is in frequency) that it has been defined as an objective measure.