Stocks: Group of resources or inputs into a system. In a bathtub, the stock is the amount of water in the tub.
Flows: Movements between stocks. A bathtub has two flows, the tap into the tub, and the drain letting water out. The amount of water is the result of the interaction between the in-flow and the out-flow.
Reinforcing Feedback: Change happen in loops, not lines. These loops are feedback forces that work with the stocks and flows. Reinforcing feedback(growth force) happens when the stocks in a system are increasing. In the bathtub, the growth force is the water coming out of the faucet, increasing the stock of water in the tub.
Limiting Factor: Systems will collapse if the growth force runs unchecked. At a certain point, the system hits a limiting factor. In the bathtub example, the limiting factor is the desired level of water in the tub. Limiting factors in a system by looking are
Max Capacity of the stocks
Goals of the actors in the system.
Balancing Feedback: Balancing feedback kicks in when system approaches its limiting factors. In a bathtub, its someone turning off the water flow when its full. If there is a goal, there will be forces that are trying to achieve that goal by balancing the system. Systems resist change thru balancing feedback that always tries to return the system to equilibrium.
Equilibrium: A balanced system is in equilibrium. There are two types...
Static: No change in system - because no inflow or outflow. If the tape is closed and the drain is plugged - no inflow or outflow.
Dynamic: No change thru equal inflow and outflow. The water in a tub was filling at the same rate it was draining.
Leverage: Forces that attempt to change a system. Different leverages have different efficacy. Some will activate the balancing feedback and lock the system. But with careful examination, you can find leverage points that's dramatic enough that a new balance point is found.