People who are born and/or raised in the Northeast or the Midwest, with those people possessing qualities of a Northerner.
Yankees are usually found in and are from the following states: Maine,
New Hampshire, Vermont,
Massachusetts, Rhode Island,
Connecticut, New York, New Jersey, Maryland, Pennsylvania, Ohio, Michigan, Indiana, Kentucky, Illinois, Wisconsin, Minnesota, Iowa, and Missouri.
The Mason-Dixon Line plays a very slight role in determining who is and isn't a Yankee, because the Mason-Dixon Line was an imaginary border that was designed to legally set the boundaries of the states allowing slavery, and of the non-slavery states; not determining North and South.
The states that contain the most real yankees are all of the New England states, Pennsylvania, Ohio, and New York.