Salinity data used to trace water movement or compute density are normally derived from measurements of chlorinity or electrical conductivity, temperature, and pressure. The latter technique has a precision about 1 order of magnitude greater than that of a typical chlorinity titration, but both are sensitive, in different ways, to variations in the ionic ratios of seawater. Present definitions of salinity are also ion dependent, causing significant variations in the salinity-density relationship which cannot be simply expressed. In order to obtain density to an accuracy commensurate with the available precision it is best to define salinity in relation to a water mass of known ionic content so that a density correction to be applied to other water masses may be expressed as variations from a fixed standard. These corrections then appear in the form of simple additive constants for most waters, and where density difference is the important parameter, no correction is necessary within a specific water mass. The new salinity definition is based on dillution by weight of a conductivity ratio labeled standard seawater. It would be invariant under compositional variations and in accord with the proposed new equation of state (Grasshoff, 1976). It is conservative within acceptable limits, would provide a 'practical salinity scale' for use by oceanographers of all levels of sophistication, and would greatly facilitate data comparisons between institutions. The present variety of computational procedures for in situ data reduction would be replaced by one set of definitive equations that would not be subject to change as the precision of physical or chemical measurement improved. A great part of the data base necessary to write these equations exists, and the remainder should be available by 1978.