Posek (Hebrew: פוסק [poˈseq], pl. Poskim, פוסקים) is the term in Jewish law for "decider"—a legal scholar who decides the Halakha in cases of law where previous authorities are inconclusive or in those situations where no halakhic precedent exists.
The decision of a posek is known as a psak din or psak halakha ("ruling of law"; pl. piskei din, piskei halakha) or simply a "psak". In Hebrew, פסק is the root implying to "stop" or "cease"—the posek brings the process of legal debate to finality. Piskei din are generally recorded in the responsa literature.