A composite sample is a sample that includes a discrete series of samples based on a temporal or rate based frequency. The composite sample could be a minimum number of samples over a time that water is flowing or over a specific period of time (e.g. 24 hours). Composite samples give a better picture of what is happening for a larger body of water, stream, or even air. When environmental scientists take samples there is a lot of time and thought that goes into making sure that sample will accurately reflect the quality of water.
40 CFR Part 136 reviews the proper sampling techniques for a variety of environmental matrices: http://www.tn.gov/environment/fleming/docs/wwt_epamethod136.pdf
Amber M. Baylor 