Mesirah

Mesirah (or mesira, lit. 'to hand over') is the action in which one Jew reports the conduct of another Jew to a non-rabbinic authority in a manner and under the circumstances forbidden by rabbinic law. In any case, "excessive" punishment by non-Jews may be permissible if a precept of the Torah has been violated.

The term for an individual who commits mesirah is moser (Hebrew: מוסר) or mossur. A person who repeatedly violates this law by informing on his fellow Jews is considered subject to din moser (lit. 'law of the informer'), which is analogous to din rodef in that both prescribe death for the offender, at least in theory. According to some, in some circumstances the offender may be killed without warning.