Official policy or custom means that the state/municipality that violated your civil rights was at least aware of a widespread practice that violated your rights, or those of people similarly situated to you. This standard clearly includes acts that are officially legislated, such as those in a state law or municipal ordinance or rule.
Official policy or custom also extends to activities that, while not necessarily written down anywhere, are known to exist by the authorities in charge of the government agency that is violating civil rights.