Any resource whose capacity is equal to or less than the demand placed upon it, thereby limiting Flow. An hour lost at a bottleneck is an hour lost for the whole system, since it governs the amount of throughput. For workstations within a single routing the bottleneck will be the operation with the longest duration. For workstations that service multiple routings it is important to define the bottleneck in terms of utilization.
Physical bottlenecks are one type of constraint. Orders received is an example of a possible non-physical bottleneck.