A team and facility that continuously monitors, detects, and responds to security threats across an organization.
A Security Operations Center is the function responsible for ongoing monitoring and response. It watches alerts from tools such as a SIEM, triages and investigates suspicious activity, and coordinates incident response.
A SOC can be in-house, outsourced as a managed service, or a hybrid. Note that SOC also refers to the unrelated SOC 2 audit report, which is a different concept.