Definition
A content delivery network is a globally distributed set of caching servers that serve content from a location near the requesting user instead of from the origin server. Because the dominant cost of a request over long distances is round-trip time rather than processing, moving bytes closer to the user is often the single largest latency win available to a system.
A CDN does two jobs that are worth separating in an interview. The first is latency: an edge server in the user's region answers in tens of milliseconds where the origin would take hundreds. The second is origin protection: a high cache hit ratio means the origin sees a small fraction of total traffic, which is what makes a viral event survivable. Modern CDNs also terminate TLS, absorb volumetric attacks, and increasingly run compute at the edge.
How it comes up in interviews
"Put a CDN in front of it" is not an answer by itself — the follow-ups are what gets graded. Be ready for cache invalidation (versioned URLs beat purge APIs), what is cacheable versus per-user, the cold-cache thundering herd on the origin, and why dynamic or authenticated responses need different treatment from static assets.