A relay service is an intermediary that forwards requests from a customer to an upstream API provider and then returns the provider’s response. Instead of calling the API directly, the client sends traffic through the relay, which may handle authentication, routing, rate limiting, caching, or billing.
In cyber security, relays matter because they can hide the real source of traffic, concentrate sensitive data, and create another place where requests or responses may be logged, altered, or abused. Attackers can use relays to mask credential abuse, resell access, or funnel many users through one upstream account. Defenders use relays deliberately too, but they should secure them with strong key handling, usage monitoring, clear billing provenance, and data-minimizing design so the intermediary does not become a privacy or fraud risk.