What is Anathema?

Legal Definition
Anathema, in common usage, is something or someone that is detested or shunned. Its other main usage comes from the New Testament and refers to a formal ecclesiastical excommunication. However, in the Old Testament, anathema referred either to something (living or inanimate) that was consecrated or something denounced as evil or accursed and set aside for sacrificial offering. It comes from the Greek: ἀνάθεμα, anatithenai, meaning "anything dedicated".
-- Wikipedia
Legal Definition
Eccl. law. A punishment by which a person is separate from, the body of the church, and forbidden all intercourse with the faithful: it differs from excommunication, which simply forbids the person excommunicated, from going into the church and communicating with the faithful. Gal. 1. 8, 9.
-- Bouviers Law Dictionary
Legal Definition
An ecclesiastical punishment by which a person is separated from the body of the church, and forbidden all intercourse with the members of the same.
-- Black's Law Dictionary
Legal Definition
An ecclesiastical punishment by which a person is shut out from all relation with the church.
-- Ballentine's Law Dictionary