A public key is used in the cryptographic technique that allows peers on a blockchain to acquire funds in their wallets. When a public key is linked to a private key, a pair of keys is formed. The private-public key pair is used to preserve the security of the blockchain. A public key is an alphanumeric string that identifies a certain node or location.
Public-key refers to a cryptographic mechanism. It has been named public-key to differentiate it from the traditional and more intuitive cryptographic mechanism known as: symmetric-key, shared secret, secret-key and also called private-key. Public-key have a concept of involving key pairs: one for encrypting, the other for decrypting.
In other words you can say, public-key is commonly used to identify a cryptographic method that uses an asymmetric-key pair: a public-key and a private-key. Public-key encryption uses that key pair for encryption and decryption. The public-key is made public and is distributed widely and freely. The private-key is never distributed and must be kept secret.