plural mo·di operandi \ˈmō-ˌdē-, ˈmō-ˌdī-\
: a method of procedure ; especially : a distinct pattern or method of operation that indicates or suggests the work of a single criminal in more than one crime"
Straight from Merriam-Webster, folks. http://www.merriam-webster.com/dictionary/modus+operandi
The idiot that said, "<i>operandi</i> is not a Latin word" is incorrect and needs more thumbs down. That entry should be removed.
I educate people and correct mistakes in "wiki" slang dictionaries by editing and making my own entries that state corrections of previous ones, when aforementioned entries are stupid. That is my M.O.
My M.O. is humiliating people who think they know more than they do even though both they and I are anonymous.
In criminal investigations, it refers to the criminal's methods whether murder or burglary or whatever as similar methods used in separate offences which could indicate the same person was responsible.
It doesn't have to pertain to crime. It can also be used to describe a person's 'trade mark' habits.