Flow describes the state of a continuous, steady workflow in which items move through the system without interruptions or congestion. In Kanban, flow is not a side effect but the primary optimization target. All other practices — WIP Limits, Pull Principle, visualization — ultimately serve to establish and maintain flow.
The state of flow can be observed on the board. When items move steadily from left to right and no column overflows, good flow exists. When items pile up in front of the Review column, flow is disrupted and Lead Time rises. Flow Efficiency, the ratio of actual value-adding time to total lead time, sits at just five to 15 percent in many organizations. The rest is wait time. This number makes clear that the greatest leverage lies not in working faster but in waiting less.
The term has two roots: Lean Manufacturing for process flow and Mihaly Csikszentmihalyi for the psychological flow state in individuals. In the Kanban context, it refers primarily to the system-level flow.