The Congestion reference article from the English Wikipedia on 24-Jul-2004
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Congestion

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Congestion is a state of excessive accumulation or overfilling or overcrowding. This general definition is broadly correct across all uses of the word. However in specific contexts the word has a more precise definition and usage.
In telecommunication, the term has the following meanings:

  1. In a communications switch, a state or condition that occurs when more subscribers attempt simultaneously to access the switch than it is able to handle, even if unsaturated.
  2. In a saturated communications system, the condition that occurs when an additional demand for service occurs.

Sources: Federal Standard 1037C and MIL-STD-188
In medicine and pathology the term is used to describe excessive accumulation of blood or other fluid in a particular part of the body.
See the traffic congestion article for a discussion of congestion in transportation.
In electric power transmission it means the inability to supply an area with the cheapest available generation due to a binding constaint on the point to point transmission route. Some plant or equipment could be causing the constraint due to thermal overload or security limits being potentially breached.

See also