From my rusty, dusty, but ever-so-trusty Encarta Reference Library:
"Ermine, name given to various species of weasel, particularly during the winter season, when the fur of individuals inhabiting colder latitudes is white. The name is applied especially to the stoat, which inhabits temperate and subarctic regions of Europe, North America, and Asia."
Careful, my mom has a couple of stone martins around a coat collar... ;)