Why do we call it “Relaxing” an edge?

Question

In Dijkstra's shortest path algorithm and others, to examine an edge to see if it offers a better path to a node is referred to as relaxing the edge. Why is it called relaxing?

Answer 1

In general mathematically, relaxation is making a change that reduces constraints. When the Dijkstra algorithm examines an edge, it removes an edge from the pool, thereby reducing the number of constraints.

It's not horribly useful terminology, but think how cool you'll sound saying it.

Answer 2

I do not think the question is related to the mathematical concept talked about in the current accepted answer. I am basing my answer on the second edition of Cormen's (CLRS) book, specifically on chapter 24 in a section about relaxation.

Context: You are searching for the shortest path between a node s and all the other nodes. Imagine you have two nodes u and v. You already have an intermediate path for them.

relax(u,v) is a function that should be read as "relax v using u". I prefer to understand the function as "shorten v's distance to s using u". You are asking yourself if you should give up on your current path s-v in favor of transforming it into s-u-v. All you have to do to see if the distance of s-u plus the additional cost of the arrow u-v are better than the distance s-v. The picture examplifies the function

A picture from CLRS's explanation on Relaxation