There is a remarkable lack of experimental data on the conditions created during cavitation bubble collapse. Indeed, only recently has strong evidence of plasma formation been obtained during single bubble cavitation. Here we have determined for the first time the plasma electron density and the ion broadening parameter during single-bubble sonoluminescence and examined them as a function of acoustic driving pressure. We find that the electron density spans four orders of magnitude and can exceed 10×1021/cc (which is comparable to the densities produced by intense laser-induced inertial confinement fusion experiments, e.g., the NOVA ICF laser at Livermore) with effective plasma temperatures ranging from 7000 to more than 16 000 K. At the highest acoustic driving force, neutral Ar lines can no longer be used as spectroscopic reporters due to the extent of ionization and to leveling of the population of states. Accounting for the temporal profile of the sonoluminescence pulse suggests that the ultimate conditions generated inside the collapsing bubble may far exceed those determined from emission from the outer transparent region of the plasma core.