Vittra is a mythical people within the Nordic folk beliefs. The notion of Vittra is a Nordic variant of a European notion of a people, underground, who live in parallel with the people and in their vicinity. They are among those who in folklore are called "the invisible".
Vittra looks, lives and dresses like humans, and unlike many other beings in folklore, they are mortal. They live underground, or rather in a parallel reality. Vittra's roads are called Vitterstråk, and in many places there are old maps of these. It goes badly for the one who builds his house straight on a Vitterstråk, then the Vittra goes with his cattle across the house.