A lair is a hidden place where a wild animal lives and rests, like a bear's den deep in the woods or a wolf's cave tucked into a hillside. Animals choose lairs that offer shelter and safety from danger. A lion might retreat to its lair after hunting, and a fox raises its cubs in an underground lair called a burrow.

The word also describes the hideout of someone sinister or villainous. In adventure stories, the villain often schemes from a secret lair: a hidden fortress, an abandoned warehouse, or an underground bunker. When someone says “the dragon's lair,” they mean the dangerous place where the dragon lives and guards its treasure.

Unlike a simple home or nest, lair suggests secrecy and often danger. You wouldn't call a robin's nest a lair, but you might call the shadowy cave where bats hang upside down a lair. The word carries a sense of mystery and menace: entering someone's lair means venturing into their territory, where they have the advantage and you might run into trouble.