In Britain the 'Least Weasel' (Mustela nivalis) is known simply as the Weasel, & this is the original use of the word. In zoological use, 'weasel' on its own is now more usually applied to the genus. In North America it is used as a common name for a number of species. However, most literary references to weasels are in fact to the Least Weasel.

The Least Weasel is generally found in farmlands, meadows, brushy areas & woodland edges.

Like all weasels, the Least Weasel is a slender animal with a short tail & short legs, enabling it to follow its prey (mostly small rodents) into their burrows. They are the smallest beast of prey, but so agile & courageous that they are regarded as a model of carnivorous animals. They prey upon mice, house rats, water rats, moles, rabbits, chickens, birds, lizards, snakes, frogs, fish, & crabs. They also kill hares, which can be more than 100 times their own size. Their natural predators are the Long-tailed weasel, Red fox, Gray fox, snake, hawk & the owl.

It's narrow head is supported on a long neck, & the legs are short. The large eyes are black, & the ears are rounded. It's fur is ginger to a rich chocolate-russet brown in colour, brighter than that of most other weasels, with white belly fur. It is rarely more than 23 centimetres (9 in) long. Although most active at night, weasels are sometimes seen during the day.

Least Weasels are highly solitary, & even mating does not occur without a fight. Females can breed several times in a year when food is plentiful. A litter of weasels numbers eight. The mother is very fond of her little blind offspring & nourishes them long after they can see. Perhaps because of their small size, Least Weasels have an even greater reputation for ferocity than the other weasels & they are much more blood-thirsty than the ferret.

One who noiselessly approaches the hiding-place of a weasel may easily secure the pleasure of watching it. He may then hear a slight rustle of leaves & see a small, brown creature gliding along. As soon as it catches sight of a human being, it stands on its hind legs to obtain a better view. The idea of running away seldom enters a weasel's head as it looks at the world with a pair of bold eyes & assumes an attitude of defiance. Men have been attacked by it. A naturalist once saw a large bird swoop down on a field, pick up a small animal and fly upward with it. Suddenly the bird staggered in its flight, then dropped to the ground dead. A weasel tripped merrily away. It had severed the bird's neck with its teeth.

There are many references to them in the popular cultures of different countries. Traditional Inuit lore held the Least Weasel in great respect because of its pugnacious nature, and the capture of one was regarded as an omen of good luck. In classical and medieval European mythology, it is sometimes said that the only thing which can kill a basilisk is a weasel (by which is meant Mustela nivalis), though it would be killed in the conflict as well.