The perfect crime is a crime for which the perpetrator of the crime will never be caught, or if caught then not convicted of the crime. A stricter definition of the term indicates that for a crime to be truly perfect, the crime should never be detected at all, removing all possibility that a person will ever be caught or tried for a crime. This is not necessarily any one particular crime, but can refer to any sort of criminal act that defies solution. The perfect crime typically reflects upon the criminal, and does not serve to indicate poor performance on the part of those investigating the crime.
Often the source of inspiration for crime writers, the perfect crime is any hypothetical crime that cannot be solved or even noticed. In its most loose meaning, this is a crime that once committed, cannot be solved for some reason. There are some real world examples of “the perfect crime” including cases in which the evidence in a case hinged upon deoxyribonucleic acid (DNA) that indicated the perpetrator was one of a pair of identical twins. Since the evidence could not be used to distinguish which twin committed the crime, when both twins have maintained their innocence there was little ability to prosecute one of them.
More elaborate or creative types of perfect crimes are often thought of by writers creating crime fiction; such a crime is often used to challenge an investigator acting as protagonist. This can include a “locked room” murder, in which a person is found dead in a room, often clearly the victim of murder, but the room is locked from within. Some researchers or investigators apply a stricter definition to the term, and for them only a crime that remains undetected constitutes a perfect crime. While this means that only the criminal would be aware that the crime was committed perfectly, it would be impossible to solve since it would remain unknown.
A perfect crime is typically thought of as indicative of the brilliance or machinations of a criminal. This type of crime does not go unsolved due to mistakes or failures made on behalf of an investigation, but rather in spite of the work that goes into solving the case. A perfect crime is, in essence, so thoroughly planned and carefully committed that no evidence exists that can indicate who has committed the crime, at least not conclusively. In reality, such crimes tend to be quite rare, and most criminals make numerous mistakes while committing a crime that provide investigators with evidence with which to find them.