bottleneck

Short definition from Wiktionary

Noun

1. 1
   The narrow portion that forms the pouring spout of a bottle; the neck of a bottle.
2. 2
   The narrow portion that forms the pouring spout of a bottle; the neck of a bottle: A portion of a bottleneck placed on the finger and used as a guitar slide.
3. 3
   In traffic, any narrowing of the road, especially resulting in a delay.
4. 4
   The part of a process that is too slow or cumbersome.

Verb

1. 1
   To slow by causing a bottleneck.
2. 2
   To form a bottleneck.