Public domain software: Software that is not protected by copyright and is freely available for use, modification, and distribution by anyone. Public domain software encourages innovation and collaboration without the restrictions typical of licensed software, although some software labeled as "public domain" may actually be under permissive licenses rather than truly in the public domain.

Public key: A publicly disclosed key that is part of a cryptographic pair, which includes a private key. The public key is used to encrypt data or verify digital signatures, while the private key decrypts the data or creates the signature.

Public key certificate: A digital certificate that verifies the identity of the certificate holder using a public key. It is part of a public key infrastructure, essential for establishing secure communications over the internet.

Public Key Cryptography (PKC): A cryptographic system that uses a pair of keys, a public key and a private key, to encrypt and decrypt messages. It is fundamental for secure communication, digital signatures, and authentication protocols. Examples include RSA, ECC, and Diffie-Hellman algorithms.

Public Key Cryptosystem: A cryptographic system that uses pairs of keys - one which may be disseminated publicly to encrypt data (public key) and another which is known only to the owner (private key) for decryption. In a public key cryptosystem, anyone can encrypt a message using the receiver's public key, but only the holder of the paired private key can decrypt it, thereby ensuring confidentiality. Additionally, these systems can be used for digital signatures, where a message is signed with a sender's private key and can be verified by anyone having access to the sender's public key, thus providing authentication and non-repudiation. Well-known examples of public key cryptosystems include RSA, DSA, and ECC (Elliptic Curve Cryptography). Public key cryptography is foundational to many aspects of digital security and is widely used in securing communications, data, and transactions over the Internet.

Public Key Infrastructure (PKI): A framework that enables secure electronic transactions through the use of a paired public and private cryptographic key. PKI ensures secure user authentication, digital signatures, and encrypted data transmission, providing a foundation for trust in digital communications.