That definition is the legal definition of narcotic; pharmacologically, narcotics are the class of drugs that have a depressive effect on the central nervous system. (Not marijuana for instance) Most are opioids such as heroin or morphine and make you sleepy, hence the root narco- as in narcolepsy. Cocaine, however, is for some reason classed pharmacologically as a narcotic, though I'm pretty sure its the only one that's more of an 'up' than a 'down'...