The first room you enter after coming from outside where you can take off your snowy or muddy shoes, get out of wet jackets, etc. and not track dirt and water into the main part of the house. I think people up here call it a breezeway. But unlike my breezeway, it's totally weather-tight and heated like the rest of the house. Some mud rooms have the washer and dryer there for convenience.
Ok, we call ours "utility room."