A Greek word meaning “devoted (to destruction), accursed” — Paul uses it in Galatians 1:8 (“let him be anathema”). Church councils attached anathemas to their definitions: to teach the condemned error was to place oneself under the council's formal censure and outside the communion.
An anathema targets the teaching and excludes the unrepentant teacher; it is the council's way of drawing a hard boundary around the faith.