a group of respondents in a quantitative study, representing a given group/population, and selected at random from that population - E.g.  representative sample of Poland's adult population, representative sample of users of a given product.
A representative sample is build in line with statistical rules for sample selection. This provides for credible and reliable generalization of results for the whole population - results from a representative sample of, with set probability and precision, are regarded as describing the actual state of affairs within the studied population.