Modus operandi (often used in the abbreviated forms MO, M.O. or simply Method) is a Latin phrase, approximately translated as "mode of operation." The plural is modi operandi ("modes of operation"). It is used in law enforcement to describe a criminal's characteristic patterns and style of work. It is also applied in fraud investigation when talking about behavior patterns that indicate specific types of fraud, e.g., "False identity is a key MO of retail banking sleeper fraud".
The term is also commonly used in English in a non-criminal sense to describe someone's habits or manner of working, the method of operating or functioning.
A criminal's MO may also be used in offender profiling, where it can also be used to find clues to the perpetrator's psychology.
The distiction between MO and signature, is in the intent. Actions considered consistent with MO can be linked to one of three intended purposes; 1) to ensure the offenders success in committing the crime; 2) to obscure or otherwise protect the offender from being readily identified or; 3) to facilitate or effect the offender’s escape. Actions discovered during an investigation which fall into one or more of these categories can be ascribed as MO.
A criminal's "signature" (an unnecessary addition or "flourish" when committing a crime) may be concealed within the or revealed as a part of the MO - the mere act of making the unnecessary addition can become part of the criminal's MO. In fact, that could be the only repeatable part of the criminal's MO, that s/he leaves a "signature", intentionally or unintentionally. Examples of property crime signatures may include acts of property vandalism or defacement of property after a burglary.
Caution: It is the professional opinion of many investigators that signature acts are often dismissed as unimportant or nonsensical. It is erroneous to assume an act held no purpose, simply because it could not be logically explained by others; to the offender the purpose was valid and necessary. In short, simply because it makes no sense to investigators does not mean it makes no sense at all.