What is the name of the magical powder used to travel via fireplaces?

The magical powder used to travel via fireplaces is Floo Powder.

In the wizarding world, Floo Powder connects fireplaces to the Floo Network, allowing witches and wizards to travel between linked locations. A traveler throws the powder into a lit fire, steps into the green flames, and clearly announces a destination.

Floo Powder is also useful for communication. A witch or wizard can place their head—or, in some cases, another part of their body—into a fireplace to speak with someone at another connected fireplace. This makes it different from a Portkey, which transports people through an enchanted object, and from Apparition, which is personal magical teleportation.

The Floo Network is regulated by the Ministry of Magic. In Britain, it provides access to many official locations, including Ministry offices. The powder is notably associated with green flames, but the destination must still be spoken correctly; unclear pronunciation can result in an unintended arrival.