Effective Population Size
Effective population size (Ne) is the size of an idealized population that would experience genetic drift at the same rate as the observed population. It is almost always smaller than the census population size, often dramatically so.
A population of 10,000 individuals where only 100 reproduce has an effective size closer to 100 than 10,000. Variance in reproductive success, sex ratio imbalance, and non-random mating all reduce Ne. The result: drift is stronger than headcounts suggest. For allele frequency changes and neutral substitutions, Ne is the number that matters, not the number you count in a census.