concept

Steady state

The condition in which a reservoir's contents stay constant not because nothing is happening but because inflow exactly matches outflow. The distinction is easy to miss and matters enormously: Earth's atmosphere holds a nearly fixed amount of water vapour while any individual water molecule stays about nine days. What is constant is the number, not the contents. The practical consequence is that whenever a concentration is said to have persisted over a long span, one is entitled to ask immediately what source is replacing it, and at what rate.