1. (removal time) The time that a given substance remains in a particular compartment of a biogeochemical cycle.

2. The time during which water remains within an aquifer, lake, river, or other water body before continuing around the hydrological cycle. The time involved may vary from days for shallow gravel aquifers to millions of years for deep aquifers with very low values for hydraulic conductivity. Residence times of water in rivers are a few days, while in large lakes residence time ranges up to several decades. Residence times of continental ice sheets is hundreds of thousands of years, of small glaciers a few decades.

3. The average time a particular element of sea water spends in solution between the time it first enters and the time it is removed from the ocean.

4. The average time that a water molecule or particulate pollutant spends in the atmosphere. For pollutants (e.g. dust from a volcanic eruption), the residence time may range from a few weeks in the lower troposphere to several years in the upper stratosphere, before it is scavenged out by precipitation. For water molecules the overall average is believed to be 9–10 days.