A subpopulation is a group that we are interested in, defined by a subset of units in the population.

a population that is part of a larger population

a subset of the training data which is expected to contain additional regularities - for instance, pages from the same site, or spans from the same document

a group of polar bears with a home range independent of but overlapping that of other polar bears

In the case that the responses are not based on the entire sample, a description of the portion of the sample whose responses are being reported appears here (e.g. women, or those who favor a given policy).