Economics, business,
accounting, and related fields often distinguish between quantities that are
stocks and those that are
flows. These differ in their
units of measurement. A
stock variable is measured at one specific time, and represents a quantity existing at that point in time (say, December 31, 2004), which may have
accumulated in the past. A
flow variable is measured over an interval of time. Therefore a flow would be measured
per unit of time (say a year). Flow is roughly analogous to
rate or
speed in this sense.