The U.S. Northern states do not, confusingly, contain all of the states in the northern half of the country. Instead, for historical reasons (mainly involving the American Civil War), the Northern states are only those which lie in the northern half of the country east of the Mississippi River. Those states in the northern portion of the West are specifically referred to as the Pacific Northwest, those states which lie in the northern reaches of the middle of the country, beyond the Upper Midwest, are Plains or Intermountain states.
The states of the North are, from east to west: Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, Connecticut, New York, New Jersey, Pennsylvania, Ohio, Indiana, Illinois, Michigan, Wisconsin and Minnesota. (Iowa is iffy.)
See also: U.S. Southern states, New England, U.S. Northeast, List of regions of the United States.