Swim
To Swim is to traverse water or some other liquid.
The swim speed of a creature, character, object or machine is the measure of ability of that entity to traverse water-like liquids. Swim speed is measured in 'world units' or 'game units'. A common movement grid is one square being five (5) real-world feet.
In some game presentations (particularly those optimized for playing with miniature models of characters and creatures without reference to a game board or grid), the move, swim, and fly speeds for various denizens of Fairyland might be listed in terms of their length or space. For example, a shark's swim speed listed as length x 4 it can move a distance on the game board equal to four times the physical length of the miniature model representing it. Other examples of potential values might be 'length', 'length / 2', 'length x 2', etc.
For another example, a swimming humanoid character might have it's swim speed listed as space x 2, meaning it can move horizontally in the air roughly twice as far as it takes up when standing on the board (ie. it's own breadth or that of it's base).