What does "S" refer to in a 70s ribosome and 80s ribosome?

The ‘S’ in a 70S ribosome and 80S ribosome refers to Svedberg’s Unit, which is used to measure sedimentation coefficient. It is an indirect measure used to determine the size and density of ribosomes by measuring how fast molecules move in a centrifuge. In cells, the lower the sedimentation coefficient, the lighter the structure and vice versa.