Ceasg (keeask) or Maighdean na Tuinne ('maiden of the wave') is a Scottish mermaid of the Highland Scots. They are half-human, and half salmon. The human half resembles that of a young woman. These ceasgs were known to carry on love affairs with human males often marrying them.

The off-springs of these unions often produced great sailors and sea captains. These mermaids are reputed to lure and seduce sailors to them in which the sailors never returned home. If a ceasg was able to be captured and overcome she had the ability to grant three wishes. This was a difficult task to achieve because this was done by the destruction of her soul, which was kept safely somewhere else such as in an object or a land feature.