The Protean Charm is a charm that links several objects together through a common purpose. The charm qualifies as Transfiguration in the Hogwarts curriculum.
In 1995, Hermione Granger used this charm to link communication Galleons for the members of Dumbledore's Army. The change in shape of one faux Galleon changed the shape of the others. Ravenclaw Terry Boot commented that the charm was N.E.W.T.-level and was very impressed by her ability to perform it as a fifth-year. Hermione explained she got the idea from the Dark Mark tattoos of the Death Eaters, which were used to summon them to their master. The coins were used constantly by various D.A. members until 1996. At that time, Ron Weasley and Hermione attempted to summon help during the Battle of the Astronomy Tower, but only Neville Longbottom and Luna Lovegood responded, as they were especially vigilant members.
The Protean-charmed Galleons were used once again in 1998 when the D.A. convened in the Room of Requirement to form a resistance against Headmaster Severus Snape and the Carrow professors. Presumably, Hermione gave the master coin to Neville, Luna or Ginny Weasley.
Draco Malfoy, inspired by Hermione's coins, used a similar method to communicate with Madam Rosmerta, whom he had under the Imperius Curse as part of his plan to assassinate Albus Dumbledore.
Etymology
The English word Protean derives from Proteus, a god in Greek Mythology. Proteus was a shapechanger, able to take many forms. As a result, the word Protean has come to refer to versatility, flexibility, or an ability to assume many forms. "Protean" may also derive from "protein," the substance that bonds in links or chains.