What is "sigma-t"

Sigma-t is a quantity used in oceanography to measure the density of seawater at a given temperature. σ is defined as ρ(S,T)-1000 kg m, where ρ(S,T) is the density of a sample of seawater at temperature T and salinity S, measured in kg m, at standard atmospheric pressure. For example, a water sample with a density of 1.027 g/cm has a σ value of 27.