Mutual TLS (mTLS).

mTLS requires both the client and the server to cryptographically authenticate each other using certificates.

In standard TLS, the client verifies the server. In mTLS, the server also verifies the client. It is the cornerstone of Zero Trust Architecture. In microservices, mTLS ensures that Service A is actually Service A before allowing it to communicate with Service B. Service Meshes automate certificate rotation and enforcement.