Money laundering is the practice of disguising illegally obtained funds so that they seem legal. It is a crime in many jurisdictions with varying definitions. It is a key operation of the underground economy.
In US law it is the practice of engaging in financial transactions to conceal the identity, source, or destination of illegally gained money. In UK law the common law definition is wider. The act is defined as taking any action with property of any form which is either wholly or in part the proceeds of a crime that will disguise the fact that that property is the proceeds of a crime or obscure the beneficial ownership of said property.
In the past, the term "money laundering" was applied only to financial transactions related to organized crime. Today its definition is often expanded by government and international regulators such as the US Office of the Comptroller of the Currency to mean any financial transaction which generates an asset or a value as the result of an illegal act, which may involve actions such as tax evasion or false accounting. In the UK, it does not even need to involve money, but any economic good. Courts involve money laundering committed by private individuals, drug dealers, businesses, corrupt officials, members of criminal organizations such as the Mafia, and even states.
As financial crime has become more complex, and "financial intelligence information gathering intelligence" (FININT) has become more recognized in combating international crime and terrorism, money laundering has become more prominent in political, economic, and legal debate. Money laundering is ipso facto illegal; the acts generating the money almost always are themselves criminal in some way (for if not, the money would not need to be laundered).