macOS Sierra: certificate
A certificate is an attachment to an electronic document that allows the safe transfer of information over the Internet. Certificates are used by web browsers, mail, and texting apps.
When you communicate with a secure site, the information exchanged with the site is encrypted. This protects your login information, credit card numbers, addresses, and other secure data.
In macOS, certificates are part of your digital identity and are stored in your keychain. Keychain Access lets you manage your certificates and keychains.