A bottleneck is a phenomenon where the performance or capacity of an entire system is limited by a single or limited number of components or resources. The term bottleneck is taken from the 'assets are water' metaphor. As water is poured out of a bottle, the rate of outflow is limited by the width of the conduit of exit—that is, bottleneck. By increasing the width of the bottleneck one can increase the rate at which the water flows out of the neck at different frequencies. Such limiting components of a system are sometimes referred to as bottleneck points.
Project management 
A bottleneck in project management is one process in a chain of processes, such that its limited capacity reduces the capacity of the whole chain.
Related concepts in project management are:
Another example is in a Surface Mount Technology (SMT) Board Assembly Line with several equipments aligned, usually the common sense is driven to set up and shift the bottleneck element towards the end of the process, inducing the better and faster machines to always keep the PCB supply flowing up, never allowing the slower ones to fully stop, a fact that would be heeded as a deleterious and significant overall drawback on the process.
Bottleneck occurs in decision making process.
Four bottlenecks that may occur:
- Global versus Local
- Center versus Business Unit
- Function versus Function
- Inside versus Outside Partners
In engineering, a bottleneck is a phenomenon by which the performance or capacity of an entire system is severely limited by a single component. Formally, a bottleneck lies on a system's critical path and provides the lowest throughput. As such, system designers will try to avoid bottlenecks and direct effort towards locating and tuning existing bottlenecks. Some examples of possible engineering bottlenecks are: processor, a communication link, a data processing software, etc.
Metaphorically a bottleneck is a section of a route with a carrying capacity substantially below that characterizing other sections of the same route. This is often a narrow part of a road, perhaps also with a smaller number of lanes, or a reduction of the number of tracks of a railway line. It may be due to a narrow bridge or tunnel, a deep cutting or narrow embankment, locations on a road where speed must be kept low due to safety reasons, or work in progress on part of the road or railway.
Capacity bottlenecks are the most vulnerable points in a network and are very often the subject of offensive or defensive military actions. Capacity bottlenecks of strategic importance - such as the Panama Canal where traffic is limited by the infrastructure - are normally referred to as chokepoints; capacity bottlenecks of tactical value are referred to as mobility corridors.
Bottlenecks in software 
In computer programming, tracking down bottlenecks (sometimes known as "hot spots" - sections of the code that execute most frequently - i.e. have the highest execution count) is called performance analysis. Reduction is usually achieved with the help of specialized tools, known as performance analyzers or profilers. The objective being to make those particular sections of code perform as fast as possible to improve overall algorithmic efficiency.
Bottlenecks in max-min fairness 
In a communication network, sometimes a max-min fairness of the network is desired, usually opposed to the basic first-come first-served policy. With max-min fairness, data flow between any two nodes is maximized, but only at the cost of more or equally expensive data flows. To put it another way, in case of network congestion any data flow is only impacted by smaller or equal flows.
In such context, a bottleneck link for a given data flow is a link that is fully utilized (is saturated) and of all the flows sharing this link, the given data flow achieves maximum data rate network-wide. Note that this definition is substantially different from a common meaning of a bottleneck. Also note, that this definition does not forbid a single link to be a bottleneck for multiple flows.
A data rate allocation is max-min fair if and only if a data flow between any two nodes has at least one bottleneck link.
Population genetics 
In population genetics, a population bottleneck occurs when the effective population size, Ne, sharply decreases to a small percentage of the original. The immediate effect of a population bottleneck is to decrease genetic diversity, promoting the effects of stochastic genetic drift over natural selection. In the long-term, repeated population bottlenecks can severely decrease population fitness: deleterious alleles are able to accumulate especially where the time interval between bottlenecks does not allow for the generation of new alleles through mutation.
See also 
- Liebig's law of the minimum
- Choke point
- Performance engineering
- Performance analysis
- Theory of Constraints
- Optimization (computer science)
- Fairness measure
- Population bottleneck
- Reverse salient