The silver foxhead medallion is a ter'angreal that was given to Mat Cauthon when he emerged from the Rhuidean twisted doorframe ter'angreal. It is one of his most prized possessions, and he keeps it on or with him almost all the time, though he did once offer it as protection to Elayne Trakand and Nynaeve al'Meara, and he eventually agreed to let Elayne study it in return for a favour.
The foxhead is made of silver, and one of its eyes bears the ancient symbol of the Aes Sedai.
The foxhead will disrupt flows of the One Power aimed at the bearer. For some unknown reason it also injures a gholam on contact, burning it as if the medallion were red-hot metal. The foxhead does not, however, affect actions of the Power which are not directly aimed at the bearer. For example, although a binding formed of Air will melt away upon touching the bearer, a stone thrown by a flow of Air will strike normally.
Another example would be lightning; once formed, however many bolts, they would follow their natural course and could strike the bearer. If a person using the One Power specifically directed a bolt at someone wearing the medallion, it would still work, as lightning is merely being directed with the Power and not a construct of it.
Though Mat is unaware, a side effect of the medallion is that it protects dreams. This was not part of the intended purpose of making the medallion; it is a true side effect.
The medallion has the ability to nullify both saidar and saidin. Its first protection from saidar comes in Rhuidean when Moiraine Damodred tries to Heal Mat from a drop of saliva from a Darkhound. Other Aes Sedai have tried to hold Mat with Air, with no success. Its first protection from saidin comes in Salidar, when Aran'gar, in the guise of Halima, channels at Mat.
Elayne Trakand was able to study this medallion and reproduce it to an extent, making several flawed copies. These imperfect copies were not as powerful as the original, in that very strong weaves could somehow overwhelm the protection. Another flaw of the copies is that if the bearer is a channeler, he/she is unable to channel the True Source while in skin contact with the medallion.