Eidetic reduction is a technique in the study of essences in Edmund Husserl's phenomenology whose goal is to identify the basic components of phenomena.
[1] It involves imagining an object of the kind under investigation and varying its features.
[2] One can take for example Descartes's piece of wax (not as a mental object, but as a demonstration of the concept of reduction).
Eidetic reduction is a form of imaginative variation by which one attempts to reduce a phenomenon into its necessary essences.
The basic steps of an eidetic reduction are threefold: first, choose some specific example (e.g., Descartes' wax).