Stub Resolver
The minimal DNS client built into operating systems and applications that forwards every query to a configured recursive resolver instead of doing recursion itself.
A stub resolver is the tiny DNS client in your operating system (or in libc's `getaddrinfo`, or in a programming language's DNS library) that sends every query to one or more upstream recursive resolvers and trusts whatever comes back. It does not walk the DNS hierarchy itself; it does not cache much beyond the response TTL; it does not validate DNSSEC unless explicitly configured. The OS picks the upstream from DHCP, manual config, or a VPN. The stub is the layer that decides whether to use DoH, DoT, or plain port-53 UDP, and it is where features like systemd-resolved's per-domain routing live.