A Vanisher is an elf with the special ability to let light pass through their bodies. This allows them to turn invisible. They blink in and out of sight as they walk.
Gnomes, however, can still see them because of the pollen and dust that settles on them. This excludes Biana and Alvar who figured out a way to hide from the gnomes. Vanishers sometimes do not even realize they blink in and out of sight as they move. Alvar uses and has used his vanishing ability to kidnap people (including Sophie and Wylie) for the Neverseen. Talented vanishers such as Della and Wraith can partially vanish or vanish into any water without getting wet. Vanishers also seem to be able to flash-step; it seems like intermediate and advanced vanishers can do so. This was shown when Della appeared in the water without getting wet (Book 4: Neverseen), and Biana vanished to the other side of the room (Book 6: Nightfall). When Sophie enhances Biana she can turn two people invisible at a time(including herself)