Increasingly abbreviated to just m.o., the expression modus operandi literally translates as "mode of operations" or "method of working".
e.g. Tom's m.o. was to get all his homework done before dinner.
In the past, a modus operandi was often associated with criminals!
e.g. The thief's modus operandi normally involved a shotgun, a well-dressed distraction, and a very fast car.