Schizophrenia means having a split or multiple personality, like Jekyll and Hyde, flipping between different people inside one head. The word gets used that way all the time, as in 'the weather is so schizophrenic today', meaning changeable.
Schizophrenia is not a split personality. It is a serious mental illness characterised by disruptions to thought and perception: symptoms can include hallucinations, such as hearing voices, delusions, disorganised thinking, and difficulty telling what is real, along with reduced motivation and social withdrawal. There is only one personality involved. The thing people are picturing, distinct identities taking turns, is a different and much rarer condition called dissociative identity disorder, formerly known as multiple personality disorder. The confusion partly comes from the name itself: 'schizophrenia' was coined from Greek roots meaning 'split mind', but its creator meant a splitting or fragmenting of mental functions like thought and emotion, not a split into separate people. So the everyday use, the 'schizophrenic weather', is doubly wrong and, for the roughly one in a hundred people who actually live with the illness, a tired and stigmatising mix-up. One person, one identity, a brain that is struggling to sort signal from noise.