A narrow portion forming the pouring spout or neck of a bottle, and in music, it can also refer to a portion used as a guitar slide.

(figurative) In traffic, any narrowing of the road, especially resulting in a delay.

(by extension) The part of a process that is too slow or cumbersome.

(transitive) To slow by causing a bottleneck.

(intransitive) To form a bottleneck.