ACME
Automatic Certificate Management Environment: the protocol used by Let's Encrypt and other CAs to issue and renew TLS certificates without human interaction.
ACME (Automatic Certificate Management Environment) is the protocol that lets a server prove it controls a domain and obtain a TLS certificate in a single automated workflow. The client (Certbot, acme.sh, Caddy, Traefik) generates a key pair, asks the CA for a challenge, satisfies it by serving a file at a well-known HTTP URL (http-01) or publishing a TXT record at `_acme-challenge.example.com` (dns-01), and receives the certificate. dns-01 is required for wildcard certificates and works even when port 80 is blocked. ACME is what makes free, 90-day, auto-renewed certs from Let's Encrypt operationally painless.