When something that was once against the law becomes legal, that process is decriminalization. Someone who wants to open a casino might lobby for the decriminalization of gambling in her city.
Interracial marriage was once illegal in most of the United States, and its official decriminalization didn't occur until 1967, when the Supreme Court ruled that such laws were unconstitutional. You can also use this word to describe what happens when laws are no longer strictly enforced, despite not being actually overturned. This kind of decriminalization might result in drug offenders being sent to treatment programs instead of prison, for example.