Na'vi can refer to either the Na'vi people or their language.
The Na'vi people are a race of blue-skinned humanoid aliens with vaguely cat-like features living on the moon Pandora in the Alpha Centauri system in the James Cameron's Avatar movie. Living in close harmony with Nature, they can be likened to Wood Elves or idealized portrayals of Native Americans.
The Na'vi language was developed by linguist Paul Frommer in 2005. It contains a system of cases (agentive, patientive, dative, genitive) and around 2249 vocables as of January 5, 2017. There is around a dozen speakers, from beginner to fluent.