noun

• The narrow portion that forms the pouring spout of a bottle; the neck of a bottle.
• (figurative) In traffic, any narrowing of the road, especially resulting in a delay.
  ""Permissive" working allows more than one train to be in a block section at one time but trains must be run at low speed in order to stop on sight behind the train in front. Such working is often authorised to allow freight trains to "bunch" together to await a path through a bottleneck instead of being strung out over several block sections, as would be necessary if absolute working were in force."
• (by extension) The part of a process that is too slow or cumbersome.
  "It is easy to create entries; processing the paperwork is the bottleneck."

verb

• (transitive) To slow by causing a bottleneck.
  "The merge bottlenecked the traffic every morning."
• (intransitive) To form a bottleneck.
  "The traffic bottlenecked at the merge every morning."