An adjacent network is a network position close enough to reach a service without being fully exposed to the public internet. In practice, this often means the attacker must already be on the same local segment, VPN, management VLAN, or other connected internal path. It is narrower than remote internet access, but it still lets an attacker interact with services that listen on internal interfaces.
This term matters in cyber security because many vulnerabilities are rated by how far away the attacker must be. Adjacent-network flaws usually imply a smaller attack surface, yet they can still be serious when the target is a trusted system such as an RMM agent, update service, or admin console. Defenders should segment networks, restrict management ports, and monitor internal paths as carefully as external ones, because an internal foothold can be enough to abuse weak authentication or integrity checks.