| Definition - A [controversial!] comma that some writers place before the conjunction that precedes the last item in a list.
The following usage guides recommend its use:
(1) The Chicago Manual of Style,
(2) Strunk and White's Elements of Style,
(3) the Oxford University Press, and
(4) Fowler's Modern English Usage
However, many newspaper style guides — for example, those published by the New York Times, the Associated Press, The Times of London, and the Canadian Press — recommend that you omit it.
(1) Lock, stock, and tomahawk
(Note the serial comma before the and.)
(2) My favourite sandwiches are pastrami, ham, cream cheese and jam and peanut butter.
(Note the missing serial comma: looks wrong, tons of ambiguity. This is why I use it.)