The question is that simple. It is in the subject.
Why the name "relaxation" in the SOR Gauss-Seidel iterative method?
I have googled for it without luck.
Moreover, when $\omega$ is used in Gauss-Seidel is named "relaxation" and when $\omega$ is used in Jacboi is named: "weight" weighted Jacobi method
So, something got to be special about Gauss-Seidel to deserve such a name.
I observed that there is a "relaxation" technique explained in this thesis back from 1948. I do not see a connection between relaxation in that context of mechanics (PDEs) and the relaxation asked here which is in the context of linear systems such as Jacobi and Gauss-Seidel. Hence, I do not consider this as a duplicate questions.