Real Dictionary

fountain pen
countable noun: A fountain pen is a pen which uses ink that you have drawn up inside it from a bottle.

fountain pen in American English
a pen in which a nib at the end is fed ink from a supply in a reservoir or cartridge
noun: a pen with a refillable reservoir that provides a continuous supply of usually fluid ink to its point

fountain pen in British English
noun: a pen the nib of which is supplied with ink from a cartridge or a reservoir in its barrel