noun.
A pen having as its writing point a small ball bearing that transfers ink stored in a cartridge onto a writing surface.
The Century Dictionary and Cyclopedia
noun.
An attachment to a compass or pair of dividers, fitted to one leg to form a seat over a hole when a circle is to be described about the hole.
the GNU version of the Collaborative International Dictionary of English
noun.
a pen which has a small metal ball as point of transfer of ink to paper, at the tip of a cylandrical and non-refillable reservoir of ink; -- short for ballpoint pen.