This article contains plot details from the latest book.
“She blinked out of sight, reappearing a second later knee-deep in the river. Sophie wasn't sure what was crazier, how fast Della had moved, or how she hadn't caused ripples in the water.”
—Narration, in Book 4: Neverseen, page(s) 56, Google Books
A Vanisher is an elf with the special ability to let light pass through their bodies, which allows them to turn invisible. When a vanisher is enhanced by an enhancer, light automatically passes through them, without them trying. Sophie, right after she discovers her enhancing ability, enhances Biana.
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 kind of water without getting wet. Vanishers also seem to be able to flash-step; 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).
Vanishers can do a trick called partial vanishing, causing their body to disappear while clothes are still visible. Or, when part of their body disappears, but the rest does not. Wraith uses this trick as his disguise when working with the Black Swan.