HTTPS (Hypertext Transfer Protocol Secure) is the encrypted version of HTTP, the protocol a browser and a website use to exchange information. It relies on TLS (Transport Layer Security) encryption so no one in between can read or alter data such as passwords or form entries. The browser padlock shows the connection is encrypted, not that the site is trustworthy.
Related term in this glossary: SSL/TLS
HTTP Secure, HTTPS protocol, SSL certificate, browser padlock