Sea Mither, or Mither of the Sea, is a mythical being of Orcadian folklore that lives in the sea during summer, when she confines the demonic nuckelavee to the ocean depths. Each spring she battles with her arch-enemy Teran, another spirit of Orcadian legend capable of causing severe winter storms, to gain control of the seas and the weather. Eventually Sea Mither overcomes Teran and sends him to the depths of the ocean, but the effort of keeping him confined there along with her other benevolent labours during the summer exhaust her, until in the autumn Teran takes advantage of her weakness to wrest control from her once again.
Stories of the Sea Mither and Teran are among Orkney's oldest legends, perhaps invented to explain the vagaries of weather and other naturally occurring events. In Shetland fishermen petition Sea Mither to afford them protection from the Devil.
Mither is defined in the Dictionary of the Older Scottish Tongue as the Scots variant of "mother", which may particularly reflect oral Orcadian use. The name of her opponent, Teran, is local Orkney dialect meaning "furious anger", and may be a derivative of tyrren, Norse for "angry".
Sea Mither is a spirit of summer days that quells the turbulent sea waters around the northern isles of Scotland. Shetland islanders, particularly fishermen, seek her protection from the Devil. Control of the seas is maintained by Teran, the spirit of winter, until Sea Mither arrives around the time of the vernal equinox in mid-March. Both spirits are invisible to humans. Teran is her arch-enemy and the pair fight bitterly, often for weeks as she tries to gain control. Their arguments cause gale force winds and heavy tumultuous seas as she tries to wrest control from him. Teran's screeches are carried by the howling gales as the two spirits try to oust each other. The period of the spring combat between the pair is termed the Vore tullye or the "spring struggle". Eventually Sea Mither overcomes Teran, relegating him to the depths of the ocean; inclement summer weather is caused by Teran's attempts to escape.
During summer months the Sea Mither also keeps the demonic nuckelavee creature confined, and undertakes benevolent labours: she empowers aquatic creatures with the ability to reproduce; warms and calms the seas; and instils a softer song-like quality to the gentle summer breeze. According to folklorist and Orkney resident, Walter Traill Dennison, during Sea Mither's reign in summer the conditions reported by islanders may have "tempted one to believe that the Orkney archipelago had become the islands of the blessed." But the continual work she undertakes to keep everything calm and the strain of maintaining control over Teran gradually tires her.