A powerful spell which creates a powerful whip or lasso of fire.
References from the canon
Dumbledore used a nonverbal spell during his duel with Voldemort in the Atrium that created a fiery rope emanating from the tip of his wand (OP36). A year later, Dumbledore used a nonverbal spell to create a ring of fire emanating from his wand like a vast lasso. It acted as a barrier against Inferi (HBP26). A year after that, McGonagall non-verbally caused a torch to fly out of its bracket, creating a ring of fire that filled the corridor and flew like a lasso at her target (DH30). Judging from the effects, these instances appear to be the same unnamed fire spell.