The difference between the shortest path in a network and the path that traffic between two points actually takes is defined as network stretch. It can also refer to the difference between the shortest physical path and the shortest logical path a packet being forwarded must travel. There could be a difference between the shortest physical path and logical path if the link cost between a set of hops is higher resulting in logical path being different.
Network stretch can be calculated based on comparing hop counts through a network, the metric along two paths and/or the delay along two paths among other things.
Stretch is not always bad and increasing stretch via Policy Based Routing or Traffic Engineering to push traffic off the shortest physical path onto a desired logical path is a desired outcome. In this case the post TE network stretch, the difference between the physical path and the logical path, is desirable, required and is a policy decision.
Defining and calculating network stretch can aid in finding the complexity of a network.
References: Computer Networking Problems and Solutions (2017)