A perpetrator is a person who is responsible for a crime. While a suspect may be suspected of having committed a crime, this term is used for the person who actually committed it. Usually, this term is only used for people who commit illegal acts or crimes. This term is often misused because it relies on a state of reality in which a person has actually committed an illegal act, or at least has been found guilty and legally convicted. A fallible legal system that does not always catch criminals or convict the actual perpetrators may still create perpetrators through conviction — a person innocent of a crime will still be a perpetrator if he is convicted.
In groups that deal with crime, this term is frequently misused. For instance, a police officer referring to a description produced by a witness may call the person described a suspect when, in fact, he or she is a perpetrator. One way to think of the distinction is that a crime with only one perpetrator may have several suspects even though only one person committed the act. The perpetrator is always the real criminal and may never even become a suspect over the course of investigation.
Under many systems, a person who is convicted of a crime is then considered to have certainly committed that crime. The legal system is considered definitive of the truth of a crime, and a conviction means for all practical purposes that a person has actually done the crime. Once a conviction has been made, the convict can then be considered a perpetrator, even if there are no witnesses who can verify that he or she committed the crime.
A perpetrator is a person, but the term is also an important concept. The idea that until a verdict has been made, a person may or may not have actually committed a crime is essential to many legal systems. Of course, there is a sense in which there are real perpetrators and real crimes, but this term is used largely for an idea. It is considered undeniable that for every real crime, there is a real perpetrator, whether or not he or she is ever found. Only in conviction are the idea of a perpetrator and an actual person combined in the same body.
One interesting question is whether or not there would be perpetrators without laws. One could argue that without laws, there cannot be crimes, and therefore there cannot be perpetrators. Perhaps a distinct system of right and wrong could maintain the concept of perpetrators, even without a system built around pursuing them. Even so, the term is dependent on an ordered system of rules for meaning.