TLS/SSL certificate

A signed document that binds a public key to domain names and is used when establishing a secure TLS connection.

In plain words

A certificate confirms that the connection to a site is encrypted and that the site is who it claims to be.

Practical example

When opening https://example.com the browser checks the certificate and displays a padlock in the address bar.

Where you encounter it

When setting up HTTPS on a website and diagnosing 'not secure' connection warnings.