Latency
Latency is the time delay between a request being sent and a response beginning to arrive — a component of overall response time, but distinct from it, since latency specifically measures the delay before data starts flowing, not the total time to receive everything.
Latency and response time are often used loosely as synonyms, but latency more precisely refers to the network and processing delay before the first byte of a response arrives, while response time typically includes the full round trip — request sent, processing, and complete response received.
Latency is heavily influenced by physical distance and network hops, which is why CDNs and geographically distributed infrastructure exist specifically to reduce it — no amount of server-side optimization fixes latency caused by physical distance between a user and a data center on the other side of the world.
