Unsub stands for "UNknown SUBject". It is used to refer to a suspect in a criminal investigation. The subject is usually suspected of a specific crime; serial killing, rampage killing, arson, etc. The term "unsub" is used by the BAU team as a code-name for the criminals they investigate on Criminal Minds. This process is repeated with the BAU team featured in Suspect Behavior and the International Response team featurd in Beyond Borders.