A particular form of "privilege." When this privilege is available, it works to bar testimony by the attorney against the attorney's client, but only if all required conditions are met, and only if no disqualifications shall be applicable.
A privilege (in this context) is a legal concept that is grounded upon creating or honoring a right to maintain one's personal privacy in certain relationships, and a privilege works to bar any testimony by one person against the other. The "attorney-client" is the oldest privilege recognized in Anglo-American law. It existed in the Roman Republic, and has been firmly established in Anglo-English common law for the past 500 years.