What is Incremental Static Regeneration (ISR)?
A rendering strategy that updates individual static pages after deployment, on a timer or on demand, without full rebuilds.
Definition
ISR, introduced by Next.js, keeps the speed of static generation while escaping its biggest constraint—rebuilding the whole site to change one page. Each page carries a revalidation window; after it expires, the next request receives the cached version while the page regenerates in the background (stale-while-revalidate semantics), and subsequent visitors get the fresh copy. On-demand revalidation via revalidatePath or revalidateTag lets a CMS webhook purge exactly the affected pages the moment content is published. This makes statically-served catalogs with tens of thousands of product pages practical, since pages can also be generated lazily on first request.