A lair is a underground or other enclosed place that animals use to hide themselves, while at sleep, hibernation or when they take part in reproduction. Some lair-using animals build their lairs, others use hollows which occur naturally, like caves.
In Scots, lair is also used for a burial-plot in a graveyard.
In mythology, heroes have often hunted dragons and similar mythic beasts to their lairs. Consequently, fantasy role-playing games often feature monster lairs as a common place for participants to find a treasure trove.
A hideout for a criminal or super villain is also referred to as a lair.