It's a difficult question to be clearly answered. We can define "Population Genetics" in many ways, probably because this study is deeply connected with many other fields, e.g. Genetics, Ecology, Developmental Biology, and Evolution. More strikingly, recent advances in molecular techniques are providing population geneticists a unique opportunity to understand how a nucleotide, gene, chromosome and entire genome behave in natural populations, and such an opportunity is drastically changing the appearance of "Population Genetics".

Nevertheless, it's still worth asking "What's Population Genetics?". As an answer, citations from two excellent textbooks seems appropriate for the definition of Population Gentics, because these are definitions in principle and in practice, respectively, and at the same time, in about decade ago (1989) and in a recent year (2000):

pp.1 in "Principles of population genetics 2nd edition" by D. L. Hartl and A. G. Clark (1989; 3rd edition is available now),

- Population genetics is the study of how Mendel's laws and other genetic principles apply to entire populations. Such a study is essential to a proper understanding of evolution because, fundamentally, evolution is the result of progressive change in the genetic composition of a population.

pp.1 in "A primer of population genetics 3rd edition" by D. L. Hartl, (2000),

- In its broadest sense, population genetics is the study of naturally occurring genetic differences among organisms. Genetic differences that are common among organisms of the same species are called genetic polymorphisms, whereas genetic differences that accumulate between species constitute genetic divergence. We may therefore define population genetics as the study of polymorphism and divergence.