A usual way of doing something.
A criminal who commits repeated crimes can often be identified by his modus operandi.
Modus operandi is Latin for "method of operating." Although often associated with police work and a favorite word of mystery writers, modus operandi is used in other contexts as well. For example, a frequent gambler who likes to play the horses may have a particular modus operandi for picking winners. The modus operandi of a cutthroat retailer may be to undersell competitors, drive them out of business, and then raise prices afterwards.