The long-tailed weasel, also known as the bridled weasel, masked ermine, or big stoat, is a species of mustelid distributed from southern Canada through much of the United States and Mexico, southward through all of Central America and into northern South America. It is distinct from the short-tailed weasel, also known as a "stoat", a close relation in the genus Mustela that originated in Eurasia and crossed into North America some half million years ago; the two species are visually similar, especially the black tail tip.

The Mustelidae are a diverse family of carnivorous mammals, including weasels, stoats, badgers, otters, martens, grisons, and wolverines. Otherwise known as mustelids, they form the largest family in the suborder Caniformia of the order Carnivora with about 66 to 70 species in nine subfamilies.