I'm so grateful that I was given a Hebrew-Hebrew dictionary, as it helps me clear up the difference between צלע and צד. It seems that צלע has a primary definition meaning rib bones, but that it also has a second definition, where in engineering it can mean "the side of a rib such as in a triangle, rectangle, and so on". However צד simply means side; this can mean direction (left/right), or good side/bad side, etc. just as the word is versatile in English; it also means side in the geometry sense, as the third of five definitions offered is essentially "every place that isn't in the center". צלע also appears as part of the phrases צלע הגבעה (hillside) and צלע ההר (mountainside). They may be somewhat interchangeable, with צלע appearing to be slightly more formal. Hopefully this is able to help someone -- I'd love it if a native speaker could either confirm or clarify this! I don't see where any particular shapes are attributed to one or the other -- can a circle have a צלע and a צד, or does only one apply to shapes not having a straight line?