What is double jeopardy?
The most basic understanding of double jeopardy is that it refers to prosecuting a person more than once for the same offense.
Double jeopardy is prohibited by the Fifth Amendment to the United States Constitution, which reads:
"No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a grand jury, except in cases arising in the land or naval forces, or in the militia, when in actual service in time of war or public danger; nor shall any person be subject for the same offense to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation."
Many states have codified a similar prohibition on double jeopardy into their constitutions. However, even those that have not done so around bound by this clause of the Fifth Amendment, which the Supreme Court has ruled also applies to the states. The Court held in Benton v. Maryland (1969) that:
"The double jeopardy prohibition of the Fifth Amendment, a fundamental ideal in our constitutional heritage, is enforceable against the States through the Fourteenth Amendment."
Legally speaking, the concept is somewhat narrower than this simple explanation alone might suggest to those of us who are not deeply involved in the legal profession.
For instance, if you are convicted of car theft, the prohibition on double jeopardy does not mean you can't be prosecuted in the future for theft of another car. It does not mean you cannot be prosecuted for another instance of theft of the same car again in the future. It does not mean that you cannot be prosecuted for violating additional laws at the same time as a result of the exact same actions that put you in violation of the law against car theft.
Once you are acquitted or convicted of a specific instance of violating the law against car theft, however, you cannot be prosecuted (or punished again, if convicted) on that same charge by the same government for that same instance of violating that law.
(There are also other boundaries that limit the protection of the prohibition on double jeopardy that are addressed in more detail elsewhere in this FAQ.)