The Protean Charm (incantation unknown) is a charm that links several objects together through a common purpose.
In 1995, Hermione Granger used this charm to link fake coins in order to communicate with the members of Dumbledore's Army. A change in one fake Galleon (in this case, a master coin) would be magically reflected in the others so that when Harry transfigured his coin to show a new date for a D.A. meeting the transfigurative change would be carried over to all the others, which would also emit heat as a signal that they were being changed by the charm to conform with Harry's. The members would therefore be alerted to any "announcement" of a new date for another D.A. meeting. 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 brands 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.
It is speculated that when Lord Voldemort branded Death Eaters with the Dark Mark, he used the Protean Charm. Though this is not for certain, it is supported by the fact that when a Death Eater touches his or her mark, the marks on other Death Eaters begin to burn in a way similar to that of the coins used by Dumbledore's Army and darken (the D.A. coins also change in appearance, though through engravings, not colour, when activated).
The English word Protean derives from Proteus, a god in Greek mythology. Proteus was a shape-shifter, 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" is also similar to "protein", derived from the same root, meaning a variable, flexible substance which forms strong bonds between its constituent parts.