In chemistry, a Svedberg unit or svedberg (symbol S, sometimes Sv) is a non-SI metric unit for sedimentation coefficients. The Svedberg unit offers a measure of a particle's size indirectly based on its sedimentation rate under acceleration (i.e. how fast a particle of given size and shape settles out of suspension). The svedberg is a measure of time, defined as exactly 10−13 seconds (100 fs). For biological macromolecules like ribosomes, the sedimentation rate is typically measured as the rate of travel in a centrifuge tube subjected to high g-force. .
https://en.wikipedia.org/wiki/Svedberg