Why did Husserl call an unchanging essential feature an invariant?

Edmund Husserl used the term "invariant" to describe an essential feature because it represents the identical element that remains constant throughout the process of eidetic reduction.1

To identify the essence of a phenomenon, Husserl proposed a method involving the systematic variation of examples in the imagination. By forming a multiplicity of variations of a given object or experience, one can focus attention on what does not change across these different iterations. The essence is defined as that "identical something" that continuously maintains itself during this process of variation; because this structure is unchangeable and persists despite the shifting factual details, Husserl labeled it the invariant.21