Slit

The slit for an hooded nib

Is usually called "slit" the cut that bisects the tines on the tip of the nib, and allows the ink that comes from feeder to flow by capillary action up to the tip. The left figure shows very clearly the slit on the nib of a Aurora 88.

The slit is usually made after depositing the iridium tip, and generally extends to the air hole, although there are nibs without air hole, where the slit simply stops at the right height, or rigid nibs for accounting where the cut is very short and stops before it, and the air hole is placed in position behind. Finally, there are also nibs with more than a slit, like the the so-called music nibs in which to provide enough ink feed to a very large tip, the tip itself is divided in three parts by two slits.