A stock is an accumulation measured at a point in time, while a flow is a rate measured per unit of time that changes the stock. Cash is a stock; income and spending are flows. A backlog is a stock; arrivals and completions are flows.
A stock rises when total inflows exceed total outflows and falls when outflows exceed inflows. Because stocks preserve the history of past flows, they create inertia, buffers, and delays.
Why it matters
A declining harmful inflow does not guarantee that the harmful stock is declining. The outflow must exceed the inflow. This distinction prevents errors in climate, capacity, finance, workforce, and inventory decisions.
Read Stocks and Flows in System Dynamics for examples and modeling guidance.