A JavaScript script that runs in the background of a browser, separate from any page, and acts as a programmable network proxy between the page and the network. Service workers power Progressive Web App features: offline support (via the Cache API), background sync, and push notifications. They have no direct SEO impact — Googlebot disables service workers when crawling — but a service worker that caches stale content can confuse repeat visitors and complicate cache busting. Service workers must be served from HTTPS and registered explicitly from page JavaScript.