A phrase used in police work, latin for "mode of operation."
The "MO," as it's abbreviated, is what the perpetrator does to commit his or her crime. It's an extremely important concept in criminal investigation. It is learned, and can change to suit the crime.
In many judicial systems, the only way to link different crimes together is through the MO. If there were a string of killings, all women, where the assailant shot from a blitz attack from behind, usually around the same type of day, then you could be led to think that the cases are connected, they have generally the same MO.
For example, there was a bank robber in Grand Rapids, Michigan. He would go in, wave his gun around, and force everyone in the bank to undress. He did it so the witnesses would be preoccupied and embarrassed, and have less chance of identifying the robber in a police lineup later on.
This was the robber's Modus Operandi, as it helped him accomplish his crime. If he went on a string of robberies, he would probably use the same tactic, it didn't fail before, so it shouldn't fail on the next bank. However, once he gets caught, there is a clear line of evidence linking all the cases, what are the odds another robber would do the same thing, and look similiar, and drive a similar getaway car?
Try not to confuse this with a criminal's signature; as it's similiar but a different concept. There was a different real-life bank robber who would make everyone in the bank undress, but then snap photos of them. It didn't help his robbery; it slowed him down and made it more likely he would get caught, but he did it for pleasure. That's signature.
Anecdote taken from "Mindhunter" by John Douglas