Negligence is a failure to do something that is expected of a reasonable and prudent person. For example if a security guard is knocked unconscious, he can not be blame, but if he is drunk, he might be considered negligent. There is always presumption of a reasonable standard of behaviour. Often this is used to describe blameworthy behaviour of professionals.

The definition of standard applicable to the offence is often quite difficult to arrive at. Often this is best judged by other professionals (conflict of interest issues?).