Definition
Plain language
Microsoft's internal security team that triages and patches reported vulnerabilities.
As stated in the literature
Microsoft Security Response Center — the body that receives, validates, and coordinates fixes for security vulnerabilities reported against Microsoft products; the entity that confirmed and assigned CVEs for the slyp-discovered Windows COM races.
Why it matters: A trusted intake-and-coordination body is what lets external researchers report serious vulnerabilities responsibly instead of dumping them publicly.
For example, a researcher reports a Windows COM race condition to MSRC, which validates it and assigns a CVE before issuing a patch.