What is "mesirah"

Mesirah (or mesira, ) 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. This may not necessarily apply to reporting legitimate crimes to responsible authority, but does apply to turning over a Jew to an abusive authority, or to a legitimate one who would punish the criminal in ways seen as excessive by Jewish community, though "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 (HE:מוסר) or mossur. A person who repeatedly violates this law by informing on his fellow Jews is considered subject to "Din Moser" (law of the informer), which is analogous to " Din rodef" in that both prescribe death for the offender, and according to some, in some circumstances he may be killed without warning.