Residence time
The residence time is the typical time of permanence of a particle in a given environment; more precisely, the decay time Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \tau} that appears in the exponential decay Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \exp(-t/\tau)} at long times. Historically, the concept was introduced to describe the time during which two particles are close to another (Ref. 1), but it can also be employed in other situations, such as in diffusion at interfaces.