TLS
also called Transport Layer Security, SSL
The protocol that gives a network connection encryption, integrity and server authentication, underneath HTTPS and most other secure transports.
Three properties, and it is worth keeping them separate. Confidentiality: nobody in the middle can read the traffic. Integrity: nobody can modify it undetected. Authentication: the client verifies the server is who the certificate says, via a chain to a trusted root — and, with mutual TLS, the server verifies the client too.
Architecturally the decisions are about where TLS terminates. Terminating at the load balancer is simplest and leaves traffic in plaintext behind it, which is only acceptable if that network is genuinely trusted. Terminating at the service, or using mTLS between services, is what zero-trust requires and what service meshes automate.
Certificate expiry remains one of the most common causes of self-inflicted outages. Automate renewal, and alert on time-to-expiry rather than on failure.