(1) Originally, a number giving the amount of chlorine, Cl, in rams per kilogram, in a sample of sea water. (2) It was later defined to be the number Cl(o/oo) giving the mass, in grams, of atomic weight silver just necessary to precipitate the halogens in 0. 3285233 kg of a sample of sea water. (3) It is now related to salinity, S(o/oo) by the defining formula S(o/oo) ≡ 1. 80653 Cl(o/oo) in which the salinity is expressed in parts per thousand. In practice, chlorinity is seldom determined chemically (e.g., by titration against silver nitrate) but is determined by measuring the conductivity of sea water and calculating the chlorinity from this.