A modus operandi (often shortened to M.O.) is someone's habits of working, particularly in the context of business or criminal investigations, but also more generally. It is a Latin phrase, approximately translated as method or mode of operation.
The term is often used in police work when discussing crime and addressing the methods employed by the perpetrators. It is also used in criminal profiling, where it can help in finding clues to the offender's psychology. It largely consists of examining the actions used by the individual(s) to execute the crime, prevent its detection and/or facilitate escape. A suspect's modus operandi can assist in their identification, apprehension, or repression, and can also be used to determine links between crimes.
In business, modus operandi is used to describe a firm's preferred means of doing business and interacting with other firms.
The plural is modi operandi. The word operandi is a gerund in the genitive case, "of operating"; gerunds can never be pluralised in Latin, as opposed to gerundives. When a noun with an attribute in the genitive is pluralised, only the head noun normally changes, just as in English with "of": "a fact of life, two facts of life".