High Quality Content by WIKIPEDIA articles! Fault, as a legal term, refers to legal blameworthiness and responsibility in each area of law. It refers to both the actus reus and the mental state of the defendant. The basic principle is that a defendant should be able to contemplate the harm that his actions may cause, and therefore should aim to avoid such actions. Different forms of liability employ different notions of fault, in some there is no need to prove fault.