WordNet (r) 3.0 (2006):

middle west
    n 1: the north central region of the United States (sometimes
         called the heartland or the breadbasket of America) [syn:
         Midwest, middle west, midwestern United States]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "Middle West":
   Dixie, Dixieland, Down East, East, East Coast, Middle Atlantic,
   New England, North, North Central region, Northeast, Northwest,
   Pacific Northwest, South, Southeast, Southwest, Sunbelt, West,
   West Coast, Yankeeland, eastland, northland, the Coast, westland,
   wild West