A Mid-Atlantic accent is specifically developed to sound neutral among American and British accents. Such accents used to be usual among the upper social classes of many cities in eastern North America, especially New York City (where it is sometimes called the "Locust Valley lockjaw" after the home of American President Franklin Roosevelt, who spoke with such an accent), Boston (where it is sometimes called a "Harvard accent" or "Boston Brahmin accent"), Philadelphia (where it is sometimes called a "Main Line accent"), and Baltimore. In fact, it formed a required segment of the education to be had at most of America's most prestigious schools until as late as 1950. In Britain, it has been most often affected by businessmen, intellectuals, and members of academia who seek or support a special Anglo-American relationship in areas of culture and commerce.
Mid-Atlantic accents have been popular in the entertainment industry. On the television program, "Frasier," the characters Frasier and Niles Crane speak with a Mid-Atlantic accent developed in the American city of Seattle. The iconic American actor, Katharine Hepburn, also spoke with a Mid-Atlantic accent developed in Connecticut, a short distance northeast of New York City. Another actor and icon, Cary Grant, spoke with a different Mid-Atlantic accent developed in Bristol on England's west coast.
One of the following
a) A British person who has lived in North america for long enough to start speaking with an accent that is on first sounding american (although there is a slight hint that they are British when thier accent slips), but when in the company of his/her fellow countryman/woman will start to speak properly. Usually, these people did not have strong accents in the first place.
b) A north american who has emigrated to the UK and has started to sound like the local people, but still has a slight twang.
Mid-Atlantic accents suck. Very much. They give me a headache.