A passive sampling or active sampling strategy over a relatively long time, during which the amount of analyte(s) extracted is proportional to its average (mean) concentration over that time.
Source: PAC, 2016, 88, 517. 'Glossary of terms used in extraction (IUPAC Recommendations 2016)' on page 526 (https://doi.org/10.1515/pac-2015-0903)