--- title: "CDN & Caching Glossary — every term that matters, defined — warmup.rocks" description: "Plain-English definitions of the terms behind CDN caching: ETag, Cache-Control, s-maxage, stale-while-revalidate, cache keys, PoPs, origin shield, tiered caching, eviction, purging and more." canonical: https://warmup.rocks/glossary/ --- [Term ## ETag An HTTP response header that identifies a specific version of a resource, so clients and CDNs can revalidate a cached copy without re-downloading it. ](https://warmup.rocks/glossary/etag)[Term ## Cache-Control The HTTP header that tells browsers and CDNs whether a response may be cached, by whom, and for how long — the primary control surface for all HTTP caching. ](https://warmup.rocks/glossary/cache-control)[Term ## s-maxage A Cache-Control directive that sets the freshness lifetime for shared caches (CDNs, proxies) only — overriding max-age there while browsers keep following max-age. ](https://warmup.rocks/glossary/s-maxage)[Term ## stale-while-revalidate A Cache-Control extension that lets a cache serve an expired response instantly while it refreshes the copy from the origin in the background. ](https://warmup.rocks/glossary/stale-while-revalidate)[Term ## Age header An HTTP response header showing how many seconds a response has been sitting in cache — the quickest way to tell whether a response came from cache and how fresh it is. ](https://warmup.rocks/glossary/age-header)[Term ## Vary header An HTTP response header that tells caches which request headers change the response — each combination of those header values gets its own cache entry. ](https://warmup.rocks/glossary/vary-header)[Term ## Cache key The identifier a cache uses to store and look up a response — typically built from the URL plus selected request attributes. Two requests hit the same cache entry only if their keys match exactly. ](https://warmup.rocks/glossary/cache-key)[Term ## TTL (time to live) How long a cached object is considered fresh before the cache must revalidate or refetch it — set via Cache-Control, CDN rules, or defaults. ](https://warmup.rocks/glossary/ttl)[Term ## Point of presence (PoP) A physical data center where a CDN terminates user connections and serves cached content — each PoP typically maintains its own independent cache. ](https://warmup.rocks/glossary/point-of-presence)[Term ## Origin shield A designated CDN cache layer between the edge PoPs and your origin — edge misses go to the shield first, collapsing many origin requests into one. ](https://warmup.rocks/glossary/origin-shield)[Term ## Tiered caching A CDN topology where edge PoPs fetch misses from designated upper-tier PoPs instead of going straight to the origin — fewer origin requests, better hit ratios. ](https://warmup.rocks/glossary/tiered-cache)[Term ## Cache eviction The removal of objects from a cache before their TTL expires, usually because the cache needs space for more popular content — the reason a long TTL is a ceiling, not a guarantee. ](https://warmup.rocks/glossary/cache-eviction)[Term ## Cache purge Explicitly removing content from a CDN's caches before it expires, typically after a deploy or content update — the counterpart to waiting for TTL expiry. ](https://warmup.rocks/glossary/cache-purge)[Term ## Cache hit ratio The share of requests answered from cache instead of the origin — the single most telling CDN metric for speed, cost, and origin resilience. ](https://warmup.rocks/glossary/cache-hit-ratio)