HTTP Strict Transport Security

Description
HTTP Strict Transport Security (HSTS) is an opt-in security enhancement that is specified by a web application through the use of a special response header. Once a supported browser receives this header that browser will prevent any communications from being sent over HTTP to the specified domain and will instead send all communications over HTTPS.

Examples
Example of the HTTP strict transport security header Strict-Transport-Security: max-age=60000

Links
HSTS Spec

Wikipedia.org entry

MDN Docs for HSTS

OWASP TLS Protection Cheat Sheet