Congestion
Beginner
A traffic jam of wires: a spot where more wires need to pass than there is room for.
Novice
More wires needing to pass through an area than the metal layers there have room for. Tools divide the chip into tiles, estimate how many wires each tile needs, compare that with how many fit, and show over-full tiles as hot spots on a heat map.
Expert
Routing demand above routing capacity, counted per routing tile (GCell) and per layer and reported as overflow. During placement it is estimated (RUDY or a quick global route). Typical causes: high local cell or pin density, macro corners and narrow channels, and clusters of high-fanout or many-pin cells.
Explained in Placement (Design Flow).
See also: RUDY, Utilization (placement density).