What is immunity in law?

Immunity for witnesses

• Use and derivative-use immunity — the testimony, and anything found through it, cannot be used against the witness. This is the federal standard, and it is enough to override the Fifth Amendment privilege, so the witness must then testify or face contempt.
• Transactional immunity — broader, and used in some states: no prosecution at all for the transactions described.

Immunity for officials

• Absolute immunity — judges for judicial acts, prosecutors for advocacy functions, legislators for legislative acts.
• Qualified immunity — protects officials, most visibly police officers, from civil suits unless they violated a clearly established constitutional right. This is the doctrine at the centre of most current US debate on police accountability.
• Sovereign immunity — governments cannot be sued without their consent. Consent is given, within limits, by the Federal Tort Claims Act and state equivalents, and those statutes bring short notice deadlines.

Other forms

Good Samaritan immunity for volunteer rescuers; overdose immunity for people who call 911; charitable immunity, still surviving in a few states; and diplomatic immunity under the Vienna Convention.

What immunity is not

It is not a promise that nothing will happen. Immunised testimony that is false still supports a perjury charge, and immunity granted by one sovereign does not always bind another.