What does Husserl mean by an invariant in eidetic reduction?

In Edmund Husserl’s phenomenology, an invariant refers to the essential, universal, and unchangeable structure of an object that remains identical throughout the process of eidetic reduction.12

To identify this invariant, the philosopher employs a method known as free variation. Starting with a concrete object, one imaginatively varies its different aspects while holding the object in the imaginative sphere. During this process, the philosopher focuses on what persists across the multiplicity of these variations; the "identical something" that continuously maintains itself despite these changes is the essence, or the invariant.12

By isolating this invariant, the philosopher moves beyond contingent or accidental features to grasp the eidos—the necessary structure of the thing itself. This process is not an empirical generalization or an abstraction, but rather a way to achieve an intuition of essences, often referred to as Wesensschau, which allows for a rigorous description of phenomena free from factual or accidental presuppositions.123