Genetic diversity is the term for the diversity of the genetic material in a population or species. More different genes inside the population means a higher genetic diversity. The gene pool is bigger.
If a species has two populations, each of them can be genetically similar within its own population but genetically very dissimilar between each population. The total genetic diversity of the species is high, but the genetic diversity of the populations is low.
Genetic diversity can be measured as;
Percentage of loci with different alleles
Amount of heterozygosity
The number of alleles for one locus present in the population
The best method is to determine the number of alleles per locus, but this is not always feasable to use as the method is more pricely than determining heterozygosity.