A sage, son of Mrikandu and Marudvati.
The story of the Vishnu tradition: Mrikandu meditated on Vishnu, who appeared to him to ask what boon he was asking for. Mrikandu had no wish, he was just glad that Vishnu appeared to him. So Vishnu decided to be born as son to Mrikandu, Markandeya.
The story of the Shiva tradition: Markandeya was supposed to live sixteen years only. However, he received from Shiva the boon of being a Chiranjivi, an eternally living one, but not immortal, therefore Vishnu spared him at time of dissolution.
After the dissolution of the universe, Markandeya found himself alone in the infinite waters. He saw a dark boy sitting under a tree. The boy offered him to rest in him, opened his mouth and Markandeya went in. He became aware of the earth, saw kingdoms, farmers, merchants, Indra, Gods and Demons, the entire universe was in this boy. He spent years in the body, found no end, no exit … until one day the mouth opened again, he stepped out and found himself under the tree, next to the boy.
Read about Markandeya here