Public-key cryptography / asymmetric cryptography, signing.
A message that is signed (encrypted) with the private key can be verified (decrypted) with the public key. Since the public key is public, anyone can verify the signature. The public key cannot create such signatures. Security depends on the secrecy of the private key.
Original illustration by David Göthberg, Sweden. 

Released by David as public domain.
Own work
Davidgothberg