Dancer is a steel sword in The Witcher 2: Assassins of Kings, added by the Enhanced Edition update. It can be found in the nekker cave while escorting Brigida during the Crown Witness quest. Along with Forgotten vran sword, Caerme, and Mourner, it is one of the best steel swords in the game. It is only accessible in chapter 3.
A sword named Dancer can also be obtained in The Witcher 3: Wild Hunt