The action or process of ceasing to treat something as illegal or as a criminal offense.
To remove or reduce the criminal classification or status of
To repeal a strict ban on while keeping under some form of regulation
As to Cannabis –
Decriminalization is the act of removing criminal sanctions against an act, article, or behavior. Decriminalization of cannabis means it would remain illegal, but the legal system would not prosecute a person for possession under a specified amount. Instead, the penalties would range from no penalties at all, civil fines, drug education, or drug treatment.
Legalization of cannabis is the process of removing all legal prohibitions against it. Cannabis would then be available to the adult general population for purchase and use at will, similar to tobacco and alcohol.