Any substance that alters brain function and alters mood or perception would be considered psychoactive. Cannabis has a multitude of cannabinoids, one of which is THC. THC is the compound in marijuana responsible for the plant being psychoactive. THC starts out as THCa but when the plant is heated, it decarboxylates into THC and that is when you the high or sense of being stoned, from that THC being ingested. It is possible to consume cannabis without THC, hence with no psychoactivity.