I don't think a real defining line exists. Generally, psychosis involves delusions (false beliefs about the world, like one is being monitored by the police), hallucinations (usually auditory or visual, oftentimes hearing voices that don't seem to come from inside), distortions (things that you know aren't real, though they might appear to be, like seeing things you know aren't real), disruptions in speech, agitation (perhaps the most dangerous). These are (basically) the "positive" symptoms. "negative/cognitive" symptoms sometimes associated are impaired ability to socialize, flat affect, depression, trouble concentrating.