In statistics, a stratum (plural strata) refers to a subset (part) of the population (entire collection of items under consideration) which is being sampled.
Stratification thus consists of dividing the population into strata within each of which an independent sample can be chosen. The process of stratification may be undertaken on a geographical basis, e.g. by dividing up the sampled area into sub-areas on a map; or by reference to some other quality of the population, e.g. by dividing the people in a town into strata according to gender or into three strata according to whether they belong to upper, middle or lower income groups.