Generally, the idea that the sovereign or government is immune from lawsuits or other legal actions except when it consents to them. Historically, this was an absolute doctrinal position that held Federal, state, and local governments immune from tort liability arising from the activities of government. These days, the application of sovereign immunity is much less clear-cut, as different governments have waived liability in differing degrees under differing circumstances.
Sovereign immunity is treated in two places in the US Constitution. Article III, Section 2 is applicable to questions involving the immunity of Federal officials from lawsuits, suits against the Federal government by a state and vice versa, and suits against the Federal government generally. The division of power between various possible sovereigns -- the state and Federal governments -- is dealt with by the Eleventh Amendment, which discusses suits between states, between states and the Federal government, and so-called diversity cases between citizens in different states. The issues are complex, and the line of Supreme Court decisions in this area is confusing and contradictory.
The Federal Tort Claims Act is the principal means by which the Federal government has waived its own liability from sovereign immunity.