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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.