A narrowing that reduces the flow through a channel WordNet 3.0 & Open English WordNet

In military strategy, a choke point, or sometimes bottleneck, is a geographical feature on land such as a valley, defile or bridge, or maritime passage through a critical waterway such as a strait, which an armed force is forced to pass through in order to reach its objective, sometimes on a substantially narrowed front and therefore greatly decreasing its combat effectiveness by making it harder to bring superior numbers to bear. Wikipedia

A constriction in a conduit or transport system, e.g. a narrowing of a road Wikipedia Disambiguation