The variables that are stated per unit of time, such as per hour, per week, per month, or per year, are referred to as flow variables.
The value of these variables do not depend upon a particular incident or event, but rather depends on a period of time.
Flow variables include income, expenditures, production, consumption, and interest, just highlighting only a few.
Final Answer: A flow variable are those variables, the quantity of which is defined over a period of time.