Certificate
In plain terms
A digital document binding an identity to a public key.
Going deeper
A certificate, signed by a trusted authority, vouches that a public key belongs to a specific entity, enabling trust in TLS. It proves who you're talking to.
Under the hood
X.509 format; browsers trust certificates chained to known Certificate Authorities, with expiry and revocation.
Examples
TLS/SSL certificates, HTTPS, code signing, Let's Encrypt, X.509.
Think of it like...
Like a notarized ID card proving a key really belongs to the named owner.