Real Dictionary

magnum
countable noun: A magnum is a wine bottle holding the equivalent of two normal bottles, approximately 1.5 litres.

magnum in American English
a wine bottle holding about 1.5 liters, about twice as much as the usual bottle
of or pertaining to a cartridge having more explosive force than an ordinary cartridge of its size would
adjective: equipped with a larger charge than other cartridges of the same size
noun: a large wine bottle having a capacity of two ordinary bottles or 1.5 liters (1.6 quarts)

magnum in British English
noun: a wine bottle holding the equivalent of two normal bottles (approximately 52 fluid ounces)