Formal description of the fraction of genetic loci within a population that consist within individuals of two different alleles.
The greater a population's genetic variation then the greater will be its average heterozygosity, at least so long as random mating is occurring. Similarly, the greater a population's average heterozygosity then generally the greater the genetic variability within that population.
Note that here average has number of loci found in the denominator, that is, it is the fraction of individuals that are heterozygous at a given locus averaged over all loci. In practice, though, this would be confined to those loci for which heterozygosity has actually been determined. The more loci included in this calculation then the greater the sampling. Therefore, at least potentially, the closer to the true average heterozygosity that will have been determined.