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" 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.