The Lady of the Lake is a name or a title used by several either fairy or fairy-like but human enchantresses in the Matter of Britain, the body of medieval literature and mythology associated with the legend of King Arthur. She plays several important roles in many stories, including providing Arthur with the sword Excalibur, eliminating Merlin, raising Lancelot after the death of his father, and helping to take the dying Arthur to Avalon. Different sorceresses known as the Lady of the Lake appear concurrently as separate characters in some versions of the legend since at least the Post-Vulgate Cycle and consequently the seminal Le Morte d'Arthur, with the latter describing them as a hierarchical group, while some texts also give this title to either Morgan or her sister.

Excalibur is the mythical sword of King Arthur that may possess magical powers or be associated with the rightful sovereignty of Britain. Traditionally, the sword in the stone that is the proof of Arthur's lineage and the sword given him by a Lady of the Lake are not the same weapon, even as in some versions of the legend both of them share the name of Excalibur. Several similar swords and other weapons also appear within Arthurian texts, as well as in other legends.