Gene flow is the transfer of genetic variation from one population to another. If the rate of gene flow is high enough, then two populations are considered to have equivalent allele frequencies and therefore effectively be a single population.
The movement from one region or place to another which can cause gene flow.
Hybridization: The process where individuals from two different species breed together and create hybrid offspring.
Population Genetics: The study of how selective forces change the genome over time including concepts like gene flow.