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A lightweight copy of the real page that React uses to figure out what changed — like making edits on a draft before writing the final copy.
An in-memory tree of JavaScript objects representing the UI. React compares the new virtual tree with the previous one (diffing) and applies only the minimal set of changes to the real DOM (reconciliation).
The virtual DOM is a representation layer that decouples the declarative component model from imperative DOM operations. React's fiber reconciler performs incremental, interruptible diffing using a work-in-progress tree, assigning priority lanes to updates. The O(n) heuristic diff assumes elements of different types produce different trees and uses keys to match children, trading theoretical optimality for practical performance.