Hmmm, such as simple question, with no simple answer, like so many words.  I understand what a "side" is.  I know it when I seen a side, but I cannot come up with a simple definition.  Merriam-Webster has (roughly) 25 definitions of "side" see here.  I suppose definition number 2 is most applicable: "2: a place, space, or direction with respect to a center or to a line of division (as of an aisle, river, or street)".  I do not think that it needs to be defined in the article. Most English speakers will understand it. Those that cannot, we can  and do  provide pictures and examples. Do we need better pictures?