How is the term for a structure that has no use in an organism?

The term is vestigial.

It is the arrangement and order of the parts within a whole. It can also be understood as a system of coherent linked concepts, whose objective is to specify the essence of the object of study.

The structure is the set of elements that characterize a certain area of reality or system. The structural elements are permanent and basic, they are not subject to circumstantial or circumstantial considerations, but they are the essence and the raison d'être of the same system.

The elements that make up a structure are defined by some basic or characteristic features, and they are differentiated or individualized from each other by what we call distinctive features. There will be distinctive features that will allow us to isolate groups, groups between groups and individuals between groups. This concept is applicable to all sciences, and among them to social sciences, where they allow analysis of the groups that integrate them and the dynamics they can generate.