Is called flow the circulation, the transfer of a given amount of persons, vehicles, information, goods carried by communication means.
When this quantity is related to a time unit, this gives a rate of flow. The rate of flow is thus a measure of circulation that varies according to the type of flow under study.
For example, road traffic is measured in Passenger Car Unit (PCU) per hour, transfer of information through the Internet is measured in bauds (per second), water conveyance in cubic metre per second, etc.
The term flow is often associated with the term network as far as the latter constitutes, in most cases, the needed support for circulation or transfer (hydrographical network, transport network, etc.).
See also: gravity model