Definition
A schema registry (or tracking plan) is the canonical list of every event your product emits and what each payload should look like. Segment Protocols, Avo, and Mixpanel's Lexicon are common implementations. The point is to make the taxonomy explicit instead of implicit.
Tracking plan vs. Skene
A tracking plan declares what should fire. Skene validates that the calls in your repo still match what should fire after the next PR. The two are complementary: a tracking plan without code validation drifts; code validation without a tracking plan can flag drift but cannot say whether the drift was intended at the design level.