A solid is an object that is 3 dimensional. A filled polygon has no thickness and thus not a solid. Solids and surfaces, though not a solid, can have their opacity controlled using the "Shade Options" palette after "Transparency" is checked.
When I use the term "polygon" in reference to this program, I'm talking about "Smart Polygons". If you set the rectangle tool to "Single Line" and draw a rectangle the inspector in the title bar will call it 4 line. But do the same thing with "Smart Polygon" selected and the Inspector now says "1 Polygon". Also be aware that if you fill a "Single Line" rectangle a new "Smart Polygon" is created on top of the four lines and it is this polygon that is filled.