1. A group of individuals of the same species that live within a defined area and interact with each other. The nature of a population is determined by such factors as density, sex ratio, age structure, birth and death rates (see reproduction rate), emigration, and immigration. The interaction of individuals is affected by how they are distributed within their habitat, i.e. their dispersion. Compare community.
2. The total number of individuals of a given species or other class of organisms in a defined area, e.g. the population of rodents in Britain. One method of estimating the size of a population of mobile animals is the mark-recapture method. See also metapopulation.
3. In statistics and experimental design, the complete set of entities from which a sample is drawn for study or experiment with the aim of making valid inferences about the population as a whole. See sampling.