Question About Structural Notation

I was wondering what the 'S' means when describing subunits? I am currently taking an immunology class and when reading about immunoglobulins it says that IgG's are monomers (7S immunoglobulins) and that IgM is a pentamer (19S immunoglobulin)? Also various other descriptions about enzyme structures describe things in term of a number followed by S. Another example is given below from the following site

"In Eukaryotes, the co-efficient of ribosomes are 80s, of which is divided into 60s for the large, and 40s for the small subunit. The 60s contain 28s rRNA, with a small fragment that is attached noncovalently and can be released upon heating; a 5.8s, and a very small - 120 nucleated of 5sRNA" http://www.cs.stedwards.edu/chem/Chemistry/CHEM43/CHEM43/Ribosomes/Ribosome.HTML

I can only assume the S stands for subunit or some other arbitrary designation. Anyone know for sure? Please no guesses, I hate it when people are unsure and just share their opinions about what it COULD be or tell you to read/look at some other resource rather than just giving a knowledgeable, straight up answer. Thanks for your time guys!