Learn in the textbookPKI and the Core of Public Key Infrastructure
DefinitionPKI is the whole system in which a third-party certificate authority uses a digital certificate to vouch that a given public key really belongs to the person who claims it.
ExplanationA public key carries no owner's name, so receiving the key alone does not let you tell it apart from an impostor's key. A certificate authority (CA) verifies the applicant's identity and then issues a certificate that binds the public key to its owner. PKI means the whole infrastructure made up of certificate authorities, certificates and the procedures for checking them, not the name of a particular product.
AnalogyAnyone can have a name stamp made, but once an official office certifies that the stamp is registered to you, others can trust it is really yours. PKI is that registration system.
Memory hook"Infrastructure" is the word for society's groundwork, such as roads and water mains.