The term Bottleneck is used to describe a resource whose capacity is less than or equal to the demand for that resource. Constraints can be assimilated to the bottleneck concept, in that they can limit the Throughput of a flow, and, at the system level, the whole performance of an organizational system.
Definition via a hydraulic example:
Imagine a hydraulic system comprised of successive pipes, each a different section.
The output flow of the hydraulic system is limited by the weakest or narrowest section, which creates a throttling effect: let’s call this the neck section.
In order to increase the flow of the system, increasing the capacity of sections that are before or after the neck section will have no effect, since the flow will be limited by the narrowest section. To increase the flow, it is necessary to focus all efforts on the bottleneck section, since it determines the performance of the entire system.