A bottleneck is literally the neck of a glass or pottery bottle. An hourglass has a bottleneck at its mid-point whose diameter governs the time that granular contents of a given mass will take to pass through.
Metaphorically a bottleneck is a section of a route with a carrying capacity substantially below that characterising 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, or work in progress on part of the road or railway.
More generally, a bottleneck is one process in a chain of processes, such that its limited capacity reduces the capacity of the whole chain. A related concept is critical path. In computer science, some authors refer to the von Neumann bottleneck.
"Bottleneck" is also a common term used for playing Slide Guitar, because the broken-off neck of a bottle was and is often used to play that style of guitar. This musical form is most popular among Blues guitarists but is also found in some country and rock playing.