Paranoiac has two jobs. As a noun it means a person who suffers from paranoia — someone who believes, without good reason, that others are watching them, following them, or plotting against them. As an adjective it means the same thing as 'paranoid': showing that kind of unreasonable fear or suspicion. The important thing is how formal and clinical it feels. Paranoiac is the doctor's word, or the old psychiatry textbook's word. In everyday life almost nobody says it. People say 'he's paranoid' for the adjective, and 'he's paranoid' again where the noun would go — English speakers just do not reach for the noun much. When paranoiac does appear now, it is usually in writing that wants to sound serious and technical, or in literary criticism and film reviews describing a mood ('a paranoiac atmosphere'). There is also a softer everyday use of the whole paranoia family that you should know: people say 'I'm being paranoid' about very small worries — checking three times that the door is locked, feeling sure the teacher looked at them strangely. That is a joke about themselves, not a medical claim. But paranoiac, being the formal word, does not stretch that way comfortably. It stays heavy.

妄想症患者；（形容詞）多疑的、被害妄想的。日常台灣人講的「他很多疑」「想太多」用 paranoid 就好；paranoiac 是偏臨床、書面的用詞。