Invisibility is a state in which an individual or object is unobservable using ones eyes. This state can be achieved in many different ways through the natural abilities of an individual, or through technological marvels.
Invisible paint created by Phor Tak when applied to an object it actually bent light around the object. Using this paint he (with the help of Tan Hadron and Phor Tak) he created the invisible flier called The Jhama, and later Tan Hadron would create a cloak of invisibility.
In The Forest of Lost Men are two cities, Invak and Onvak, the men of these cities have mastered a scientific means of rendering themselves invisible. Once a day, every person in the city swallows a small sphere that renders them invisible after an hour.
On Thuria the closer moon of mars the two known native inhabitants have both natural abilities. The Tarids use their mental abilities to render themselves invisible, and the Masena are able to change color to blend in with their environment (not really invisibility).