A public key is a cryptographic key that is used in asymmetric encryption. It allows users to encrypt messages or verify digital signatures without needing to share a secret key. Public keys are part of a key pair, which also includes a private key that must be kept confidential. Public keys are widely used in various applications, such as securing online communications and transactions. For example, in the context of blockchain technology, public keys help ensure the integrity and authenticity of cryptocurrency transactions, allowing users to send and receive funds securely.