What is a space/spatail harmonic?

Hi

I am doing a project on folded waveguides and I am reading some IEEE papers for literature review. I always come across this term "space harmonic" and fail to understand what it is. I have checked online but don't get it. Even posted in the Electrical Engineering section but didn't get any answers.


Could anyone explain/link me to an understandable definitation of space/spatial harmonic(s)

Thanks