In the Matter of Britain, Morgause (/ˈmɔːrɡoʊz, -ɡɑːz/ MOR-gohz, -⁠gahz), also known as Anna, Gwyar, Belisent, or simply as the Queen of Orkney, is a queen and member of King Arthur's family, usually his aunt or sister.

In the early chronicles and romances based on or inspired by Geoffrey of Monmouth's Historia Regnum Britanniae, as well as in the Welsh tradition, she is named as Gawain's mother, the daughter or sister of Uther Pendragon, and either the full or half-sister of King Arthur. She has also been named as the sister of Morgan le Fay, or combined with her with into a single character. In most cases, she is the wife or widow of King Lot, and rules over either Orkney, Lothian, or Scotland. However, her name varies between texts and traditions, as does the issue of her children other than Gawain.

In a later popular tradition, Mordred is the offspring of Arthur's own accidental incest with his estranged half-sister, whom Thomas Malory's seminal Le Morte d'Arthur calls Morgause. Additionally, he names Knights of the Round Table Agravain, Gareth, and Gaheris as her sons, the last of whom later kills her.

In Geoffrey of Monmouth's early 12th-century Norman-Welsh chronicle Historia Regum Britanniae, she is named Anna, and is depicted as the only daughter of Uther Pendragon and his wife Igraine, thus making her King Arthur's full younger sister. She is named as the wife of King Lot, with whom she is the mother of Gawain and also presumably Mordred (the text describes him as Arthur's nephew without ever mentioning any siblings besides her). However, Geoffrey says very little about her otherwise. Her relationship with Lot is later elaborated on in the romance De Ortu Waluuanii, which describes how a teenaged Lot fell in love with Anna when he was a royal hostage serving as her page at the court of Uther Pendragon.

In the Brut, Anna and Lot, king and queen of Scotland, had five daughters and two sons: Gawain and Mordred. Norman poet Wace's chronicle Roman de Brut names her Gawain's mother and queen of the Scots, even though Lot is not truly a king there. However, it does not mention her relation to Mordred (again described as Arthur's nephew).

In John of Fordun's 14th-century Scottish chronicle Chronica Gentis Scotorum, Arthur was the bastard son of Uther, making Anna and her son Mordred the rightful heirs to the throne. This motif also appears in later Scottish narratives, including Hector Boece's Historia Gentis Scotorum. In this version of the story, Lot is king of the Picts with Anna (later called Cristina) as his queen. Here, she is also depicted as Uther's rightful heir, but as Uther's sister— and Arthur's aunt— instead of his daughter.

In Alain Bouchart's Breton Grande Croniques de Bretagne, "Anna or Emine" is Uther's eldest child, who marries Budic and gives birth to Hoel (Hywel).

The earliest known form of the Morgause name is Orcades (Norcadés), given to her in the First Continuation of Chrétien de Troyes' Perceval. It is likely that her name was originally a place name, as "Orcades" coincides with the Latin name for Scotland's northern Orkney islands, the lands often described as being ruled by Gawain's parents. Medievalist Roger Sherman Loomis suggested that this toponym was corrupted first into the variants of "Morcades" and finally into "Morgause" due to the influence of the name "Morgan," and that her character was derived from the goddess Deichtine. She appears as Morcades (Morchades) in the early 13th-century poem Les Enfants Gauvain and Heinrich von dem Türlin's Diu Crône. A version of her character appears in the Vulgate Cycle named Brimesent (variants include Hermesent). This naming convention is adopted in the Middle English Of Arthour and of Merlin, where she is called Belisent.