Mutual TLS (mTLS)
TLS where both sides present a certificate — the client proves its identity to the server too, not just the other way around, using X.509 certificates.
aka: mTLS
Ordinary TLS only authenticates the server to the client; mTLS adds the reverse. It’s heavy to operate — every client needs a certificate and a renewal path — but hard to beat for assurance, which is why it persists in government and military smart cards, regulated B2B APIs, and increasingly machine-to-machine authentication inside service meshes.