Elementary my dear Watson.
In the words of Sherlock Holmes, this part is simple. If only at everything about notation were this elementary. A unison is only one pitch played or sung at the same time by two or more people, or the same pitch played in two different places at the same time on a keyboard instrument with more than one manual. Picture an organist playing the same C in two places at the same time. An octave or 8th sounds so much like a unison that many people can’t hear the difference. Octaves double the frequency (the vibrations per second) of the lower note, the vibrations per second, and in all but the most bizarre instances they have the same name. The octave of C is C, the octave of F# is F#, and so on.