What is the "2013 Cole Memo"?
The Cole Memorandum was a United States Department of Justice memorandum issued August 29, 2013, by United States Deputy Attorney General James M. Cole during the presidency of Barack Obama. The memorandum, sent to all United States Attorneys, governed federal prosecution of offenses related to marijuana.
The memo stated that given its limited resources, the Justice Department would not enforce federal marijuana prohibition in states that "legalized marijuana in some form and ... implemented strong and effective regulatory and enforcement systems to control the cultivation, distribution, sale, and possession of marijuana," except where a lack of federal enforcement would undermine federal priorities (such as preventing violence in marijuana cultivation and distribution, preventing cannabis impaired driving, and preventing marijuana revenues from going to gangs and cartels).
A copy of the full "Cole Memo" can be found here.