A detective is an investigator, usually a member of a law enforcement agency. Some are private persons, and may be known as private investigators or “private eyes”.
Informally, and primarily in fiction, a detective is any licensed or unlicensed person who solves crimes, including historical crimes, or looks into records.
In some police departments, a detective position is achieved by passing a written test after a person completes the requirements for being a police officer. In many other police systems, detectives are college graduates who join directly from civilian life without first serving as uniformed officers. Some people argue that detectives do a completely different job and therefore require completely different training, qualifications, qualities and abilities than uniformed officers. The opposing argument is that without previous service as a uniformed patrol officer, a detective cannot have a great enough command of standard police procedures and problems and will find it difficult to work with uniformed colleagues.