Most pens made today have what is known as a cartridge/converter filling mechanism. This means that the pen uses an exchangeable ink cartridge to store the ink as compared to a more traditional "fountain pen" that fills from an ink bottle.
However, if the cartridge is removed and replaced with a filler, the pen is "converted" to filling from an ink bottle. The device that replaces the cartridge is called a converter, because it converts a cartridge pen into a fountain pen.
See this page for a converter compatibility information.
Recent Blog articles