The team that monitors, detects, and responds to security threats — the common entry point for the SOC Analyst role.
What it means
A security operations center (SOC) is, per Microsoft, "a centralized function or team responsible for improving an organization's cybersecurity posture and preventing, detecting, and responding to threats." (NIST's CSRC glossary also defines the term.)
One job title inside a SOC is SOC Analyst. Tier 1 work is described as monitoring alerts and logs and escalating real incidents, using tools that can include a SIEM.
Definitional only; see the SOC Analyst role page for the cited skills and roadmap.