Edge Runtime is a V8-isolate-based execution environment for serverless functions that runs in CDN POPs around the world, not a single origin region. Vercel Edge Functions, Cloudflare Workers, and AWS Lambda@Edge all implement variations of this pattern.
The trade-off vs Node.js runtime: faster cold starts (single-digit milliseconds), much lower latency for the median request, but a restricted API surface (no native modules, limited CPU + memory, fetch-style I/O only). Right for: A/B testing decisions, auth checks, request rewrites, lightweight personalisation, geo-routing.
Wrong for: heavy data processing, image manipulation, anything requiring native binaries or large heap. The placement decision (edge vs node vs static) is the architectural call most teams get wrong -- defaulting everything to edge inflates cost; defaulting everything to node inflates latency.