A certificate is a digital document that verifies the identity and authenticity of a website. A certificate is issued by a trusted authority called a certificate authority (CA). A certificate contains information such as the website's domain name, the CA's name, the expiration date, and a digital signature. If a website's certificate is expired, invalid, or untrusted, the browser will warn the user that the site may not be secure and prompt them to click a link to continue. The user can check the certificate validity by clicking on the padlock icon next to the address bar and viewing the certificate details. References: The Official CompTIA IT Fundamentals (ITF+) Student Guide (Exam FC0-U61), Chapter 6, Section 6.2, Page 260.