also: robber · rat · rat in the hole · hole player · lurk
A defender sitting in the middle of the field with no assigned man, reading the quarterback and jumping a throw. Two words for nearly the same job, and coaches disagree about whether they are the same thing.
2 meanings, all below. Jump to the one you heard.
A defender, usually a safety, drops into an intermediate middle area with no man assigned to him and reads the quarterback. His job is to arrive on a throw somebody else was covering, which is where the name comes from: he is stealing a completion rather than preventing one.
What makes it work is that he is the one defender on the field playing the quarterback instead of a receiver, so he can start moving before the ball is out. What makes it risky is that everybody else is playing as though he is not there, so a robber who guesses wrong has left an area genuinely uncovered.
lurk, hole playerrat, match quarters, Tampa 2In a man coverage where the defense has one more defender than the offense has receivers, the leftover player is often placed in the middle of the field to help, and rat, or rat in the hole, is a common name for him.
The usual distinction drawn between rat and robber is what the coverage behind them is doing: rat generally names the free defender in a man scheme, and robber generally names a defender reading route concepts within a zone or match structure. Plenty of coaches use the two interchangeably, and where they do, asking what the rest of the coverage is doing resolves it faster than arguing about the word.
rat in the holerobberBoth words describe a JOB rather than a position, and either can be filled by a safety, a linebacker or a nickel depending on the call. That is why hearing the word tells you what somebody is doing and not who he is.
Rare below high school, because it needs a defender who can read a quarterback rather than a receiver, which is a genuinely different skill and one of the last ones young defenders develop.