Lashon hara, literally "evil tongue", is the prohibition on speech that damages another person's standing. Its distinctive feature, and the reason it is so demanding, is that it applies to true statements. False damaging speech is a separate and graver offence, motzi shem ra, defamation; lashon hara covers the accurate report that harms.
Its scriptural anchor is Leviticus 19:16, "you shall not go about as a talebearer among your people", which sits in a chapter that also forbids hating one's brother in the heart, requires honest rebuke, and commands love of neighbour, a context the commentators treat as defining. The narrative paradigm is Numbers 12, where Miriam speaks against Moses and is stricken, a passage Deuteronomy 24:9 instructs Israel to remember.
Rabbinic literature treats the offence with unusual severity, comparing it to the three cardinal sins together and describing it as killing three people at once: the speaker, the listener and the one spoken about (Arakhin 15b). The classic systematic treatment is Israel Meir Kagan's Hafetz Hayyim (1873), which assembled the scattered rulings into an ordered code and gave its author the name by which he is now universally known; its publication made the observance of these laws a self-conscious movement.
The prohibition is not absolute, and the exceptions are where its real work is done. Disclosure for a constructive purpose, le-to'elet, is permitted, warning someone entering a business partnership or considering a marriage, protecting a person from harm, or seeking counsel, subject to conditions that the tradition specifies tightly: the speaker must know the matter firsthand, the purpose must genuinely be protective rather than punitive, nothing may be exaggerated, no less damaging means may be available, and the harm caused must not exceed what the purpose requires. Contemporary rabbinic writing has applied the category at length to journalism, to political speech and to online communication. In the case of abuse, most contemporary authorities across the denominations have ruled that the obligation to prevent harm to others governs and that reporting is required, rejecting earlier readings that treated disclosure as forbidden speech.